Tenses: When to use two tenses and when to use the pefect?

[afvatcha]

Hi,

I have a question on when to switch between tenses (two in the same sentence) and when to use the Present Perfect and Past Perfect.

For instance in the 2007 study guide (pg. 45) the is a sample sentence:

"He IS thin now because he SPENT the last six months on an intensive diet."

Is this an example of a sentence that is considered wrong on the GMAT and you should switch from two tenses (in the same sentence) to the present perfect?

Are two tenses in one sentence always wrong on the GMAT?

[esledge]

"He IS thin now because he SPENT the last six months on an intensive diet."

Is this an example of a sentence that is considered wrong on the GMAT and you should switch from two tenses (in the same sentence) to the present perfect?

Not wrong at all! The rule is "don't change tenses UNLESS there is a clear, compelling reason to do so." The time language "now" and "the last six months" provide the justification for switching tenses.

Are two tenses in one sentence always wrong on the GMAT?

Nope! In fact, one of the other perfect tenses must ALWAYS be used with another tense.

He COMPETED in his first marathon after he HAD TRAINED for four months.

HAD TRAINED = past perfect, for the earlier of two past actions.
COMPETED = simple past, for the more recent of two past actions.