Question

How do you correct the subject-verb disagreement in the following sentence?: The gazelle run if they notice that the lions is not asleep.

Answer 1

The gazelle run if they notice that the lions are not asleep

Explanation:

This is a "Implicative/predictive" statement. It means that one fact is true if another one is true, therefore, the gazelle will run if the lions are not asleep, thus they will remain if the lions are asleep.

This kind of sentence can be tricky, one possibility is to use the present tense in the two states, the clause and the consequence.

Clause: if they notice that the lions are not asleep
Consequence: The gazelle run.

It is not wrong to use the future:

The gazelle will run if they notice that the lions are not asleep

even

The gazelle might run if they notice that the lions are not asleep
The gazelle would run if they notice that the lions are not asleep

And so on...

Independently, the verb to be should be correct.

See:

https://en.wikipedia.org/wiki/Conditional_sentence

Answer 2

The gazelle runs if they notice that the lions are not asleep

Explanation:

In English, the verb(s) should agree with the subject(s).

The gazelle -> it (used for "things")
The lions -> they (third person of the plural, personal pronoun)

For the verb to be:

I am/you are/ she, he, it is/they are/ we are;

For the verb run,which is irregular in the past:

I run/ you run/ she, he, it runs/ they run/ we run.

Just apply and we get:

The gazelle runs if they notice that the lions are not asleep.

This is a conditional sentence, something must happen, "the lions are not asleep", for the other to happen, "The gazelle runs." Otherwise the gazelle will stand still. By the way, the plural is gazelles.

If so:

The gazelles run if they notice that the lions are not asleep