Whats the difference between the expression "They do not inflect" and "They are defective"

Question

What I understand is that Defective verbs do not inflect.
Am trying to understand what the difference is between being defective and not inflecting - from reading the below wiki extract I cannot see the distinction.

Also - with modals like can/could and will/would etc - is "could" not a inflected form of "can"? (Inflection via base change to express past tense) How is that not inflection?

FROM http://en.wikipedia.org/wiki/English_modal_verbs The verbs customarily classed as modals in English have the following properties:

• They do not inflect, except insofar as some of them come in present–past (present–preterite) pairs. They do not add the ending -(e)s in the third-person singular (the present-tense modals therefore follow the preterite-present paradigm).
• They are defective: they are not used as infinitives or participles (except occasionally in non-standard English; see Double modals below), nor as imperatives, nor (in the standard way) as subjunctives.

Answer 1

The first is a statement about their morphology: forms such as *canning and *wills do not exist (in those senses).

The second is a statement about their syntax: modals cannot fit into sentence frames like I want to X or I was Xing.

These overlap, in the sense that forms in -ing are both lacking morphologically and unacceptable syntactically. But the lack of a distinct third-person singular form is purely morphological, while the lack of an infinitive is purely syntactic.

Answer 2

The first statement is about the fact that a form exists but it bears no inflectional marks.

The second statement says that some forms don't exist at all (e.g. there is no gerund for CAN).