Dear A Class:

You wrote:
> Hello there,
> How can ‘for’ be used to express purpose? What comes after ‘for’ in this
> case? Can’for +…ing’ be used instead of infinitive?
>
I am not sure that I completely understand the question, but in modern English the gerund normally follows a preposition unless the preposition already has the word “to.”

This is useful for helping people in their studies. (typical–preposition plus gerund)

He did that in order to help her read. (infinitive–since the preposition already has the word “to”)

Occasionally you may see material before the 19th century which will have the infinitive after a preposition, but that is no longer used except as noted above and would be considered old-fashioned at best, incomprehensible at worst, or used to make fun of the speaker.

There is, for example, a popular American folk song that goes, in part, “I come from Alabama with a banjo on my knee;/ I’m going to Louisiana my true love for to see” ; but this is a humorous song and part of the humor comes from the mangling of the language. Another two lines say, “It rained all night the day I left, the weather it was dry;/ The sun so hot, I froze to death, Susannah, don’t you cry.” It’s a silly song, so the language is silly. 🙂

The title of the song is “O Susannah.”

I should add that many times the infinitive by itself suggests purpose. For example, the normal way of saying that line from the song would be, “I’m going to Lousiana to see my true love.”

I hope this answers your question.