Use of Infinitive

Question

1. He gave me a pen to write with.

2. He gave me money to spend.

Why spend is not taking preposition as it is done in first example. Kindly help me.

Answer 1

The direct object of "write" is the book or letter. The method of writing uses a preposition:

He wrote a letter with a pen.

The direct object of "spend" is the money. Direct objects don't use prepositions.

He spent the money.

This is part of the grammatical structure of English. We can't write "He spent with the money". So we can't write "He gave me money to spend with". Neither sentence is idiomatic.