إذا لم تجدي مساعدة في المكتب، اتصلي بي.

Because the sentence is addressed to one woman.

In Arabic, verb forms change depending on who is being addressed. Here:

• تجدي = you find when speaking to one female
• اتصلي = call! when speaking to one female

If you were speaking to one man, you would say:

• إذا لم تجدْ مساعدةً في المكتب، اتصلْ بي.

So the -ي here is a strong clue that the listener is feminine singular.

Because لم makes the following present-tense verb jussive.

The normal form for you (feminine singular) find is:

• تجدين

But after لم, the verb becomes jussive, and with this kind of verb the final ن drops:

• تجدين → تجدي

So:

• لم تجدين = incorrect
• لم تجدي = correct

This is a very common pattern in Modern Standard Arabic.

In Arabic, لم is the standard negator used before the present tense in the jussive mood.

So structurally:

• تجدي = present-form verb
• لم تجدي = negative jussive form

A learner often expects a direct equivalent of English do not, but Arabic does not build it that way. Instead, لم + present/jussive is a normal grammatical pattern.

In this sentence, even though لم often translates like a past negation in other contexts, the whole clause is inside a condition with إذا, so the meaning becomes:

• if you do not find...

not necessarily a simple past meaning.

إذا means if here.

It introduces the condition:

• إذا لم تجدي مساعدة في المكتب = If you do not find help in the office

Then the result follows:

• اتصلي بي = call me

So the sentence has this structure:

• condition
  • result

Arabic also has إن for if, but إذا is very common for situations seen as realistic, expected, or possible in normal circumstances.

Because اتصلي is the imperative form of the verb اتصل.

The verb is:

• past: اتصل
• present: يتصل
• imperative to a woman: اتصلي

So this is not the verb وصل or to arrive. It is the verb اتصل بـ meaning to contact / call.

The command form is built from that verb pattern, which is why it begins with ا.

Because the verb اتصل normally takes the preposition بـ.

So Arabic says, literally:

• contact/call by-me or contact me via بـ

The expression is:

• اتصل بفلان = contact someone
• اتصل بي = contact me / call me

Here, بي is just:

• بـ
  • ي

So this is the normal Arabic structure, even though English uses a direct object with call me.

مساعدة is the direct object of تجدي.

So the idea is:

• تجدي مساعدة = find help

In fully vowelled Arabic, it would usually appear as:

• مساعدةً

because it is accusative as the object of the verb.

In normal unvowelled writing, those final case endings are usually not written, so you often just see:

• مساعدة

Because Arabic verbs already show the subject.

For example:

• تجدي already means you (feminine singular) find
• اتصلي already means you (feminine singular), call!

So Arabic usually does not need to add أنتِ unless the speaker wants emphasis, contrast, or extra clarity.

A full version with emphasis could be:

• إذا لم تجدي مساعدة في المكتب، فأنتِ اتصلي بي.

But in normal speech and writing, that would usually be unnecessary.

Literally, في المكتب means in the office.

In English, depending on context, you might naturally say:

• in the office
• at the office

Arabic uses في very commonly for location, and English may choose either preposition depending on style and context.

So grammatically it is:

• في = in
• المكتب = the office

Yes, Arabic word order is somewhat flexible.

The given sentence is very natural:

• إذا لم تجدي مساعدة في المكتب، اتصلي بي.

But you could also move some parts for emphasis, for example:

• إذا لم تجدي في المكتب مساعدةً، اتصلي بي.

This is still understandable, but the original version sounds smoother and more neutral for most learners.

So the sentence order you were given is a good standard model to learn.

Here are the most useful versions:

• to one man: إذا لم تجدْ مساعدةً في المكتب، اتصلْ بي.
• to one woman: إذا لم تجدي مساعدةً في المكتب، اتصلي بي.
• to more than one person: إذا لم تجدوا مساعدةً في المكتب، اتصلوا بي.

This is a good way to notice how both the verb in the if-clause and the imperative change with the person being addressed.