In connected speech we use weak forms in non-stressed syllables. - not 'for' but 'fe', not 'of' but 'ev', not 'to' but 'te' (the sound of the vowel changes to the 'schwa'). If you don't use the weak form it sounds as if you are stressing the preposition (or article in the case of 'the and 'a' or 'an'). I want A cup OF tea. I'm going FOR A walk. If you teach English to foreign learners, you teach them to use these weak forms, as it sounds unnatural not to. I don't see a problem with 'try and', although its use does seem to be restricted to the present tense.