When can preposition "by" be omitted?

Question

I came across theses sentences:

He made a fortune selling property in Spain.

He paid for dinner using his credit card.

I want to know when it's okay to omit "by".

Please give answers based in grammar and not personal feelings.

Answer 1

If you use a verb, you don't necessarily need a preposition. In both your examples, you can use one or the other:

• He made a fortune selling (verb) property in Spain.
• He made a fortune from (preposition) property in Spain.

Also:

• He paid for dinner using (verb) his credit card.
• He paid for dinner by (preposition) credit card.

In both these examples, the meaning is unaltered by using one or the other.

When you use a verb and a preposition together, it is called a prepositional verb. You only need to use these when it alters the meaning.

For example:

• I believe John
• I believe in John

These mean very different things - the first means you accept that John speaks truthfully; the second means that you have faith or confidence in John.

Also:

• I knocked the door
• I knocked at the door

The first means you accidentally damaged the door by colliding with it; the second idiomatically means that you struck against the door to attract attention.

So, simply put - you make a choice based on the meaning. In your examples, using a prepositional verb adds nothing to the sentences. If you need to know how to use a particular verb, look it up in a good dictionary and you will see examples of usage.