These words are always prepositions. However, if you want to know why some people think that the words in the first combinations are adverbs and the verbs in the second are prepositions, the reason is this.
For a long time we had a silly idea that we could define a class of words according to the word that came after it. So we defined prepositions as words that came before noun phrases. This is a bit unusual because we do not define any other types of word in this way. For example, we still say a verb is a verb, whether it comes before a noun, an adjective or another verb.
When we had a preposition which didn't come before a noun phrase, we used to say it was an adverb. In the first group of examples, the last word doesn't not have to come before a noun phrase. It's possible for the noun phrase to come before that last word, or there may not be any noun phrase at all:
- Put the meeting off
- I turned the offer down
- My car broke down
Because it is possible to have sentences with no nouns after these "phrasal verbs", the last word must be an adverb according to our old definition. We can put the noun phrase afterwards if there is a noun complement:
- Put off the meeting.
- Turn down the offer.
But this is not important. The noun does not have to come after the last word, so the old grammar says that that word cannot be a preposition. It must be an adverb.
In the second group of examples, there must be a complement. There is always a noun phrase after the last word:
- Look after your friends.
- Talk about your holidays.
- Wait for a bus.
These noun phrases have to come after the last word in the 'phrasal verb'. If we put the noun between the verb and the second part of the phrasal verb then the sentence will be ungrammatical:
- *Look your friends after. (wrong)
- *Talk your holidays about. (wrong)
- *Wait a bus for. (wrong)
Because the second part of these phrasal verbs must come before a noun, these words are classed as prepositions, in the old grammar.
In the third group, there are three words: the verb and two other words. There is always a noun phrase complement when we have a three word phrasal verb. Because the third word comes before a noun, the old fashioned grammar says it is a preposition. But the middle word doesn't come before a noun so it is an adverb - according to this old grammar:
- look forward to the party
- look (verb); forward (adverb because the next word isn't a noun phrase); to (preposition because the next word is a noun phrase); the party (noun phrase).
Of course the reality is that these extra words are prepositions in every example. There are many test we can do to show this.
What you really need to know about phrasal verbs
The important thing about phrasal verbs, is that for some of them, we can put the noun phrase between the verb and the preposition. If the noun phrase is a pronoun, then in these phrasal verbs the pronoun must go between the verb and the preposition:
- He picked it up.
- *He picked up it. (wrong).
Because of this, it is a good idea to learn phrasal verbs with a pronoun. So you need to learn:
For more information about prepositions see:
- A Student's Introduction to English Grammar, Huddleston and Pullum, 2002