Most prepositions and conjunctions have two different pronunciations: what is called a "strong form" and a "weak form".
Basically and without intending to give a course on Phonetics a preposition such as for ex: for has at least these two different pronunciations.
/fo:/ and /fə/ being the last pronunciation the one we are going to study here as it is the one used ,in rapid speech ,to join words.
Look at the following groups of words. Listen to the recording and notice how the middle word is pronounced with a /ə/ . Practise saying the phrases.
ready to go
stay at home