French subjunctive is usually triggered by certain kinds of verbs that require their complement to be in subjunctive but also by certain conjunctions; one could establish four classes of triggers:
- "verbes de volonté": verbs expressing wishes, e.g. (Je veux que tu m'ENTENDES (= I want you to LISTEN to me))
- "verbes des sentiments": verbs expressing subjective thoughts, e.g. Il est une tragédie qu'il PARTE (= It is a tragedy that he LEAVES), Je suis content que tu SOIS mon fils (= I am glad that you ARE my son)
- "verbes de la balance": verbs like penser (= to think), être sur (= to be sure) etc., depending on how likely you consider the proposition to be true: Je ne pense pas qu'elle AIT un chien, mais je ne suis pas sur (= I don't think that she HAS a dog, but I am not sure), but Mon père ne croit pas qu'elle a A un chien parce qu'il ne l'a jamais vu (= My father doesn't believe that she HAS a dog because he has never seen it, indicative instead of subjunctive)
- certain conjunctions, e.g. Mon frère regarde la télé avant qu'il AILLE au lit (= My brother watches TV before he GOES to bed), Ma copine travaille quoiqu'elle SOIT en congé (= My friend works although she IS in vacation).
French indirect speech works similar as in English: For present tense, imperfective is used; if the speech event is in the past, time shift occurs.
This results in the state that the mood used for indirect speech depends on the verb in the main clause; if it is one of the subjunctive triggers (like Il pense que... (= He says that...) it will be in subjunctive, but for all others (like Il dit que... (= He says that...)) imperfective mood, i.e. no special indication of indirect speech (with the exception of time shift).
German Konjunktiv 1 (not to be confused with Konjunktiv 2, see below) is only1 used for indirect speech, i.e. reports of what someone said (or wrote): Er sagt, er SEI der neue Nachbar (= He says that he IS the new neighbour), Sie fragte, ob das ein Problem SEI (= She asked whether this WAS a problem)).
Concerning tense, you can't really say that "tense comes before mood" because the Konj1 mood also knows different tenses (however only compound tenses: Der Pfarrer sagt, er HABE darauf GEHOFFT (= The priest says he HAD HOPED for it)), so the mood will not be supressed by tense if this is what you meant by "tense comes before mood".
As @ˈvʀ̩ʦl̩ˌpʀm̩ft said, Konjunktiv 1 occurs rarely in spoken language, there it is usually replaced by Konjunktiv 2 with würde (see below) or, most of the time, simply indicative (Peter sagt, dass er uns HILFT instead of Peter sagt, dass er uns HELFE (= Peter says that he helps us), but in written language (especially newspapers), it is good style to do it to emphasise that what is reported is not the speaker's/writer's belief but only what was said and there is no judgement made about whether this is actually true or not.
On the other hand, there are contexts where this emphasising doesn't make that much sense, as in your example sentence Ich denke, dass du da bist (= I think that you are there) where it is automatically clear that this is what you believe (because this is what the sentences explicitely says) so it would be weird to indicate this is just what someone else thinks but not your own thought, because obviously you do believe that the addressee is there so it is more natural to just use indicative.
If you were to say Ich sagte, dass er da SEI (= I said that he WAS there) however, Konj1 is somewhat more normal because this is now again just a report of what was said and it might for example be that you lied, i.e. that you don't necessarily believe the complement to be true. So you might want to use Konj1 instead of indicative here to indicate this is just what you said, but then again (esp. in colloquial speech) this could be interpreted like the complement was actually not true (because if it was, it would have been clearer by you to express this truth by using indicative, and Konj1 often sounds a bit like you didn't believe the claim to be true) - it's a little difficult.
As a rule of thumb I would say that more formal, written language requires Konj1 for all uses of indirect speech, and this is what you actually find in newspapers (especially there), so the form does have a regular use for indirect speech. But there are exceptions to its usage (esp. in the case of denken (= to think), indicative or Konj2 often fits better), and in colloquial speech, Konj1 is almost extinct or at least weird to say and replaced by indicative or Konj2 with würde instead.
German Konjunktiv 2 is used for
- irrealis in
- conditionals, both in the antecedent and in the consequent (Wenn Robert das SÄHE, WÄRE er stolz auf dich (= If Robert SAW this, he WOULD BE proud of you))
- improbable or irreal consequences (Er fuhr so schnell, dass er fast einen Unfall gebaut HÄTTE (= He drove so fast he almost MADE an accident) or Wir haben zu spät bestellt, als dass das Paket noch rechtzeitig ANKÄME (= We ordered to late for the package TO ARRIVE in time))
- irreal comparisons (Es fühlt sich an, als HÄTTE ich das alles schonmal erlebt (= It feels like I HAD already experienced all this before))
- propositions that you are unsure about (Es könnte sein, dass er schon gegangen ist (= It MIGHT BE that he already left), WÜRDE das denn funktionieren? (= Would that even work?))
- politeness (Ich HÄTTE gern ein Glas Wasser (= I WOULD LIKE to have a glass of water), KÖNNTEN Sie mir bitte helfen (= WOULD you please help me), Der Bericht WÄRE jetzt fertig (= The report is complete now, you might hear this for example a submissive secretary not wanting to disturb the boss))
- replacing Konjunktiv 1 when
- the latter is identical to indicative (Sie sagen, sie HABEN keine Angst (= They say that they HAVE no fear, could be both indicative or Konj1)) to make clear again it is only a report and not a necessary truth
- or when you express your doubt in what was said (Sie behauptet, das GINGE nicht, aber ich glaube ihr nicht so recht (= She claims that this DOESN'T work, but I don't really believe her))
Also note that regular Konj2 is often replaced by the forms with würde (er KÄME -> er WÜRDE KOMMEN (= he CAME -> he WOULD COME) especially in colloqial speech2, but also when Konj2 is identical to preterite (*Wenn das STIMMTE (= If it WERE true (Konj2) or = If it WAS true (past tense))).
Similarly as with Konj1, if you want to use Konj2 in past, you need the compound forms, here with würde and hätte: Wenn ich genug Geld BESÄßE, FLÖGE ich nach Mallorca (= If I OWNED enough money, I WOULD FLY to Mallorca) vs. Wenn ich genug Geld BESESESN HÄTTE, WÄRE ich nach Mallorca GEFLOGEN (= If I HAD OWNED enough money, I WOULD HAVE FLOWN to Mallorca), Meine Freundin dachte, es LIEFE gut (= My friend thought it WORKED well) vs. Meine Freundin dachte, es WÄRE gut GELAUFEN (= My friend thought it HAD WORKED well).
But again, I wouldn't say that "tense comes before mood" because a different tense doesn't make the conjunctive disappear. (I'm still not sure if this is what you meant.)
In comparison, this means that there is some overlap, but the distribution is a bit different:
- French uses subjunctive in the context of certain triggers, often verbs expressing personal thoughts, wishes, etc., but also certain conjunctions.
- French uses (analogously to English) indicative (with time shift for speech events in the past) to express indirect speech, as long as the sentence does not already require subjunctive, which means that some indirect speech uses (usually when something was said) go with indicicative and some (usually when personal thoughts, feelings etc. are the context) with subjunctive.
- German has a partition between Konjunitv 1 for indirect speech (both reports of what someone said and reports of what someone believes or hopes etc., but not for personal opinions like I regret that...) and Konjunktiv 2 for irrealis or similar meanings, with some exceptions when forms are morphologically ambiguous in tense/mood or when indirect speech and irrealis mix up (e.g. when a proposition that is reported to have been said is believed to be false by the reporter such as in They think that... but I don't think they are right, Konj2 can be used instead of Konj1). However, indicative often takes over the meaning of Konj1 especially in colloquial speech.
- French subjunctive and German indicative have similar meaning when a context where French requires subjunctive and German does not requie conjunctive (e.g. I regret that...) and when indirect speech is expreseed by subjuncitve in French (given that there is a trigger verb) and German indivative is used instead of Konj1 (mostly in colloquial speech).
- French subjunctive and German Konj1 can have similar meaning in indirect speech involving subjective thoughts (like He thinks that it was unfair), but French subjuncitve is used where German Konj1 would not be used (e.g. when reporting subjective feelings) and German Konj1 is used where French would use indicative (e.g. in He says that...).
- French subjunctive and German Konj2 can have similar meaning when expressing irreal statements (like He thought that it would be a good idea but it really wasn't), but French subjunctive is not always translateable with Konj2 (especially where French subjunctive is required due to conjunctions, which are never triggers of conjunctive in German) and on the other hand Konj2 is used for more kinds of meanings (especially around irrealis) than French subjunctive is.
Footnotes
1 apart from a few special and usually conventionalised, unproductive uses for wishes (Lang lebe der König (= Long live the king)) or ancient-sounding recipes (Man nehme drei Eier (= Take three eggs))
2 "Real" Konj2 is in spoken language even rarer than preterite, which is also as good as extinct in spoken language, but for preterite there are at least some words where pretierite is still perferred over (or equally preferred as) perfect tense, such as Ich dachte instead of Ich habe gedacht (= I thought), Ich meinte instead of Ich habe gemeint (= I meant) or Es ging nicht instead of Es ist nicht gegangen (= It didn't work), but in spoken language you wouldn't use the Konj2 forms Er dächte (= He would think), Er meinte (= He would mean) or Er ginge (= He would go), but the compound forms Er würde denken, Er würde meinen and Er würde gehen.
References
Cases of usage mostly taken from here and here; comparison, example sentences and additional comments by my own.
