Look at the sentence below:
He can run faster than I can.
Is there any difference in pronunciation between the first occurrence of can and the second? The answer is yes. The first can be pronounced with a weak form kən if it is unstressed. The second is not usually weak no matter what its stress status is. It is pronounced kæn.
ˈhiː kən rʌn ˈfɑːstə ðən ˈaɪ kæn
The reason is that the second can is stranded. Modal verbs like can and auxiliary verbs like have are usually followed by a main verb. In our sentence this is true of the first, but not of the second occurrence of can. The main verb has been left out here and the modal is stranded. Stranded modals and auxiliaries are always strong. Here are some more examples:
Fred likes cabbage more than Jim does. (= dʌz)
They’ve spent more money than we have. (= hæv)
She made the same mistakes as the candidates had (= hæd) before her.
Stranded words often occur at the end of a sentence, but the last example shows that this is not always the case.