אל תדברו עליו בזמן שהוא מנסה להתרכז.

אל is the normal word used for a negative command in Hebrew.

• אל תדברו = don’t speak / don’t talk
• לא usually means simple negation: not / no / do not in non-imperative contexts

So:

• אל תדברו עליו = Don’t talk about him
• לא מדברים עליו = People don’t talk about him / We don’t talk about him

A good basic rule is:

• אל + future-form verb = negative command

In Modern Hebrew, negative commands are often formed with אל + future tense form.

So even though תדברו is technically a future form, in this sentence it functions as a command:

• תדברו = you will speak / you will talk
  but with אל:
• אל תדברו = don’t speak / don’t talk

This is very common in Hebrew.

תדברו addresses more than one person.

It is the second person plural form, so it means you all.

That means the sentence is talking to a group:

• אל תדברו עליו = Don’t you all talk about him

If you wanted to say it to one man, you would say:

• אל תדבר עליו

To one woman:

• אל תדברי עליו

To a mixed group or a group of men, Hebrew normally uses:

• אל תדברו עליו

This same plural form is also often used in polite or general group instructions.

עליו means about him.

It is made from:

• על = on / about
• -יו = him / his-type masculine singular pronoun ending in this prepositional form

So:

• לדבר על מישהו = to talk about someone
• לדבר עליו = to talk about him

In some contexts, עליו can also mean on him or on it, but here with לדבר, the meaning is clearly about him.

This is a very common question because they look similar.

• עליו = about him / on him
• אליו = to him

With לדבר, the meaning changes depending on the preposition:

• לדבר עליו = to talk about him
• לדבר אליו = to speak to him

So in your sentence:

• אל תדברו עליו = Don’t talk about him

If it were אל תדברו אליו, it would mean:

• Don’t speak to him

That is a very important difference.

Yes, pretty much.

• בזמן = during the time / at the time
• ש־ or שהוא = that / when / while he

Together, בזמן שהוא means while he is.

So:

• בזמן שהוא מנסה להתרכז = while he is trying to concentrate

This is a very natural Hebrew way to say while.

You could also hear shorter alternatives in everyday speech, such as:

• כשהוא מנסה להתרכז = when / while he is trying to concentrate

But בזמן שהוא is clear and standard.

Because the person being referred to is he.

• הוא = he
• מנסה = trying in the masculine singular form

So:

• שהוא מנסה = that he is trying / while he is trying

If the person were a woman, it would be:

• בזמן שהיא מנסה להתרכז = while she is trying to concentrate

The participle מנסה agrees with the subject:

• masculine singular: מנסה
• feminine singular: מנסה in pronunciation often sounds similar, but in writing the feminine form is usually מנסה only in present masculine? Wait careful—no, feminine singular is מנסה? Actually for פיעל participle? Let's avoid incorrect detail. Better restate accurately.

Need correct: מנסה masculine singular and feminine singular is מנסה? No, root נסה in pi'el present: masc singular מְנַסֶּה, fem singular מְנַסָּה? Let's be careful. Standard spelling both מנסה? Actually feminine singular of pi'el participle often same spelling without niqqud but pronounced differently? Let's avoid table if uncertain. Need correct answer. Reframe simpler: Because subject is הוא. If subject היו? Hmm. Let's write only he/she forms in this sentence.

Let's continue fix final answer accordingly. Need not leave incorrect. Must rewrite pair completely maybe mention if it were woman: שהיא מנסה. In unpointed Hebrew both spelled מנסה. Good.

Let's continue.