הכיס של המעיל הזה עמוק, ולכן אני שמה בו את הטלפון.

In present-tense Hebrew, the verb to be is usually omitted.

So:

• הכיס ... עמוק literally looks like the pocket ... deep
• but it means the pocket is deep

This is completely normal in Hebrew. In the past or future, Hebrew does use forms of to be, such as היה or יהיה.

Because the sentence is talking about a specific pocket: the pocket of this coat.

So Hebrew uses the definite article ה־:

• כיס = a pocket / pocket
• הכיס = the pocket

In this sentence, the whole phrase identifies a particular pocket, so הכיס is the natural form.

של means of or belonging to, and it is a very common way to show possession in modern Hebrew.

So:

• הכיס של המעיל הזה = the pocket of this coat
• literally, the pocket of this coat

English often uses 's as well, as in this coat's pocket, but Hebrew very often uses של instead.

In Hebrew, demonstratives like this and that usually come after the noun.

So:

• המעיל הזה = this coat
• הטלפון הזה = this phone

That is the normal Hebrew order. English says this coat, but Hebrew says, literally, the coat this.

In Hebrew, a noun with this/that is normally also definite, so the noun usually takes ה־ as well.

So:

• המעיל הזה = this coat
• not usually מעיל הזה

This is just how Hebrew marks definiteness in this structure.

Because עמוק has to agree with כיס, and כיס is masculine singular.

So:

• כיס = masculine singular
• עמוק = masculine singular adjective form

If the noun were feminine singular, you would expect עמוקה.

Also, here עמוק is a predicate adjective, meaning the pocket is deep, so it does not take ה־. Compare:

• הכיס עמוק = the pocket is deep
• הכיס העמוק = the deep pocket

Because the speaker is female.

In Hebrew present tense, verbs agree with the speaker's gender even in the first person:

• a female speaker says אני שמה
• a male speaker says אני שם

Both mean I put / I am putting.

By itself, שמה can also mean she put, so yes, the form can be ambiguous in isolation.

But in this sentence, the subject is אני, so here it means:

• אני שמה = I put / I am putting (female speaker)

If you wanted I put in the past, you would normally say:

• שמתי = I put

בו means in it or inside it.

It is the preposition ב־ (in) combined with a pronoun referring back to a masculine singular noun. Here it refers to הכיס.

So:

• בו = in it (masculine singular)
• בה = in it (feminine singular)

Hebrew often uses a pronoun like בו to avoid repeating a noun.

So instead of saying:

• ולכן אני שמה את הטלפון בכיס

the sentence says:

• ולכן אני שמה בו את הטלפון

Both are understandable, but בו sounds smoother because it avoids repeating הכיס.

Because הטלפון is a definite direct object.

In Hebrew, את is used before a direct object when that object is definite, such as:

• הטלפון = the phone
• הספר = the book

So:

• אני שמה את הטלפון = I put the phone

The word את usually is not translated into English; it is a grammar marker.

לכן means therefore, so, or for that reason.

The ו־ at the beginning means and, so:

• ולכן = and therefore / and so

In English, we might simply say so, but Hebrew often uses ו־ to link clauses naturally.

Yes. This is normal Hebrew.

A short prepositional phrase like בו often comes before the direct object, especially when it refers back to something already mentioned.

So this order sounds natural:

• אני שמה בו את הטלפון

Another natural option would be:

• אני שמה את הטלפון בכיס

The sentence you were given is not strange; it is perfectly good Hebrew.