Weak forms occur in stress-timed languages such as English when the word itself is not stressed. This makes such words tricky for untrained listeners to identify as as they may well be expecting the stressed strong form.
The main words with weak forms in Received Pronunciation are:
a, am, an, and, are, as, at
be, been, but
had, has, have, he, her, him, his
shall, she, should, some
than, that, the, them, there, to
was, we, were, who, would
Sometimes native speakers think they are using the language badly when they use weak forms but this is not the case. They are a perfectly natural part of the English language.
Teachers, especially, should continue to use weak forms. If teachers only use strong forms when they speak to students they will be giving them a completely erroneous impression of the way the spoken language sounds, almost as bad as pronouncing the "t" in "listen". This is a very bad form of teacher talk which teachers need to be aware of and consciously fight against.