Weak forms are syllable sounds that become unstressed in connected speech and are often then pronounced as a schwa.

Example
In the sentence below the first 'do' is a weak form and the second is stressed.

What do you want to do this evening?

In the classroom
Structural words, such as prepositions, conjunctions, auxiliaries and articles are often pronounced in their weak form, since they do not carry the main content, and are therefore not normally stressed. Learners can find them difficult to hear and this interferes with understanding. Counting the number of words in a sentence, or sentence dictations can help raise awareness of weak forms.