Several weeks ago, I talked about the difference between because and because of. Basically, because is a conjunction, while because of is a preposition.

He was late because his sister was sick. (conjunction connecting 2 clauses)

He was late because of his sister. (preposition with an object – only 1 clause)

The difference is clear. If you are connecting 2 clauses (complete sentences) together you should use because. If you are just adding an object, you should use the preposition because of.

However, some words in English, such as before and after, can be used as either a conjunction or a preposition:

After he ate dinner, he studied. (after used as a conjunction)

After dinner, he studied. (after used as a preposition)

He ate dinner before he went to the party. (before used as a conjunction)

He ate dinner before the party. (before used as a preposition)

These examples are also pretty clear. Many students become confused, though, when using a gerund (verb form +ing) after the preposition.

After eating dinner, he studied. (after used as a preposition with gerund as object)

He ate dinner before going to the party. (before used as a preposition with gerund as object)

When using before or after as a conjunction, the subject must be repeated. (A conjunction connects 2 clauses.) When using before or after as a preposition with a gerund, the subject should not be repeated. (The gerund phrase modifies the existing subject.)

I was tired after I drove all day. (conjunction – subject is repeated)

I was tired after driving all day. (preposition- no subject in gerund phrase)

Before the team got on the bus, they had lunch together. (conjunction – subject repeated)

Before getting on the bus, the team had lunch together. (preposition – no subject in gerund phrase)

Don’t make this mistake.  Be careful not to repeat the subject when using a gerund phrase.

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