I'm going to try to live up to Hillel's example and try to give you an answer short enough to answer while standing on one leg: It depends on which consonants are in the root, and where they fall.
Take the ordinary "vanilla" verb כתב (to write). The masculine singular present form in the pa'al binyan is כּוֹתֵב (kotev), with feminine singular present form כּוֹתֶבֶת (kotevet). There are many similar verbs:
and so on, and their feminine counterparts are:
But there are several consonants that throw a wrench into the works:
א ה ח י ו נ ע They have subtle sounds, some of which are or were guttural (pronounced in the throat). They generally influence the infix vowels in a predictable ("regular") way, but sometimes they do something oddball, and sometimes they fall out entirely.
Let's say you have ה in the final position. The pattern for the masculine forms is similar to what it was before:
but the pattern for the feminine forms becomes:
So that's what you might call a regular irregular pattern. Whenever you have final ה, that's what you'll see.
Similarly, final ע causes a change:
with feminine forms:
(Note that פּוֹגֵעַ is the masculine singular present form of the verb in the active pa'al binyan; above, you had פַּגוּעַ, which is in the passive pu'al binyan.)
But there are some oddities. One of the verbs you chose, ישנ, is one of them. The initial י is the reason for the weirdness, but other words that begin with that letter are more regular (for example, יוֹדֵעַ, one of the verbs I listed above). Fortunately, this kind of weirdness is not common enough to hide the more regular patterns. Other oddities are the adjective-like verbs like צרכ, whose forms are quite different in the present, but I believe those can be counted on the fingers of one hand.
I really like Lewis Glinert's Modern Hebrew: An Essential Grammar, which is clear, practical, and thorough, but not scholarly. He goes into the subject of vowel changes within the binyanim in detail.
I confess that I didn't stand on one leg while I typed this, but I hope I at least answered your basic question and didn't squelch your interest in learning more.