A/the with post-modification

Hello there,
I've been tripped up in class by teaching some of the "rules" of article usage. My first mistake was saying that one should use "the" when the noun is followed by a non-partitive of-phrase, eg:
"the behaviour of monkeys"
"the growth of the plants"
vs
"a bottle of beer" (partitive)
"a number of questions"
However, this does not hold true. So why do we say:

"the behaviour of monkeys"
but
"a knowledge of physics (is necessary)"
"an understanding of the factors which..."
and
"Computer-support can provide ease of diagram generation"

Is it to do with qualities of the noun?
My second mistake was saying that we use "the" with second mention. However, it's context-dependent of course. In the passage below, 2nd & 3rd mentions use "a". Presumably this is to do with specificity - ie, the questionnaire is not specified at any point.
"In Case Study 1, which acted as a pilot study, three types of questionnaire were tried: a self-completion questionnaire, an interview-based questionnaire and a questionnaire plus a group discussion. An interview-based questionnaire was considered most appropriate for rich qualitative feedback along with obtaining an unbiased rating of each diagram. Consequently, Case Study 2 and 3 adopted only an interview-based questionnaire."
Does it all come down to some hard-to-pin-down notion of definiteness, or are there some useful guidelines I can give to my students?

Thanks,
Helen