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. Sep 21, 2017 at 23:26
  • They joined forces (in order) to fight their common enemy.
    – Davo
    Sep 22, 2017 at 11:52

1 Answer 1


'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.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .