I learnt that after a preposition, the rule is that the gerund form of the verb is used (instead of the infinitive form), as in "Thank you for coming".
In this sentence:
- They joined forces to fight their common enemy.
It seems to me that here, 'to' is used as a preposition. So, should the sentence be?
- They joined forces to fighting their common enemy
It sounds awkward to me.
Best Answer
'To' can be either a preposition or an infinitive marker.* They're hard to distinguish because they're identical in form.
'To' as a preposition is normally part of a phrasal verb:
It can also be a pure preposition expressing a direction or a way to obtain something. For example:
'To' as a preposition takes a noun phrase or a gerund as a complement. So your reasoning is right.
When used as an infinitive marker, 'to' is followed by the 'plain' infinitive form of verb. This infinitival 'to' normally introduces a clause functioning as a purpose adjunct. You can usually add 'in order' before the word 'to'.
*It's usually called a subordinator because it heads a subordinate clause. I prefer to avoid this fancy term, however.