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:

  1. 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?

  1. They joined forces to fighting their common enemy

It sounds awkward to me.

  • 1
    According to most (but not all) contemporary grammarians the to in that sentence is not a preposition but a 'infinitive marker' or 'complementizer' (depending on what theology the individual grammarian professes), required in many syntactic contexts to signal that the following plain form is an infinitive. – StoneyB on hiatus Sep 21 '17 at 23:26
  • They joined forces (in order) to fight their common enemy. – Davo Sep 22 '17 at 11:52

'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:

I'm looking foward to meeting you. ('to' is part of the idiom 'to look forward to'.)

It can also be a pure preposition expressing a direction or a way to obtain something. For example:

A guide to teaching English.

'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'.

They joined forces (in order) to fight their common enemy.

*It's usually called a subordinator because it heads a subordinate clause. I prefer to avoid this fancy term, however.

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