לא רק שהתרגול היה קצר, אלא שהוא גם היה מעניין מאוד.

It means not only ..., but also ....

In this sentence:

• לא רק = not only
• אלא = the connector used in this structure, roughly but rather / but
• גם = also

So the sentence is saying the exercise had two positive qualities:

1. it was short
2. it was also very interesting

Because לא רק is followed by a full clause.

• שהתרגול היה קצר literally means that the exercise was short
• In English, we often leave out that
• In Hebrew, after לא רק, using ש־ before a full clause is very normal

So לא רק שהתרגול היה קצר is literally something like not only that the exercise was short..., but in natural English we simply say not only was the exercise short...

Because אלא is the standard word in the fixed pattern לא רק ... אלא גם ...

A simple way to think about it:

• אבל = ordinary but
• אלא = the preferred connector in more structured contrasts like not only ..., but also ...

So:

• לא רק ... אלא גם ... = standard and natural
• לא רק ... אבל גם ... = much less standard

הוא repeats the subject, referring back to התרגול.

Hebrew often repeats the subject pronoun when starting a new clause, especially in a sentence like this. It makes the second part feel complete and clear:

• אלא שהוא גם היה מעניין מאוד = but it was also very interesting

The הוא is not there for extra meaning; it is mainly for normal Hebrew sentence structure and flow.

Because each clause has its own predicate in the past tense:

• התרגול היה קצר = the exercise was short
• הוא גם היה מעניין מאוד = it was also very interesting

In Hebrew, when you describe something with an adjective in the past, you usually use היה / הייתה / היו.

Compare:

• Present: התרגול קצר = the exercise is short
• Past: התרגול היה קצר = the exercise was short

So the sentence needs היה in both parts because both descriptions are in the past.

Because they agree with התרגול, which is a masculine singular noun.

Agreement in Hebrew means adjectives match the noun in:

• gender
• number

Here:

• תרגול = masculine singular
• so קצר and מעניין are also masculine singular

If the noun were feminine singular, you would get forms like:

• קצרה
• מעניינת

מאוד means very.

In Hebrew, מאוד usually comes after the adjective:

• מעניין מאוד = very interesting
• literally: interesting very

That word order is normal in Hebrew, even though English puts very before the adjective.

תרגול usually means practice, drill, or exercise, depending on context.

Common possibilities:

• a classroom exercise
• a practice activity
• a drill for reviewing material

So in this sentence, התרגול most likely means something like the exercise or the practice activity.

Yes. A very common alternative is:

התרגול היה לא רק קצר, אלא גם מעניין מאוד.

This also means The exercise was not only short, but also very interesting.

The difference is mostly one of structure and emphasis:

• לא רק שהתרגול היה קצר... sounds a bit more clause-based
• התרגול היה לא רק קצר... sounds a bit more compact

Both are natural, and both are useful to know.

Usually it is expected, because it matches the English also in not only ..., but also ...

So:

• לא רק ... אלא גם ... = the full, standard pattern

If you remove גם, the sentence may still be understandable, but it sounds less balanced and less complete. Using גם is the normal choice.