التبني مهم.

A common Levantine-style pronunciation is:

it-tabannī mhimm
or
et-tabannī mhimm

A few notes:

• الـ often sounds like il-/it-/el-/et- in Levantine, depending on region and surrounding sounds.
• In التبني, the ل of الـ is not pronounced because ت is a sun letter. So you hear something like it-tabannī, not il-tabannī.
• مهم in Levantine is often pronounced mhimm or mhemm, rather than the more careful Standard Arabic muhimm.

Because Arabic usually leaves out the present-tense verb to be.

So:

• التبني مهم
• literally: adoption important
• natural meaning: Adoption is important

This is very normal in both Standard Arabic and Levantine.

If you wanted was, you would usually add a verb, for example:

• كان التبني مهم = Adoption was important

التبني is a noun, specifically a verbal noun (often called a maṣdar).

It comes from the verb تبنّى, which means to adopt.
So التبني means adoption or the act of adopting.

This is similar to English pairs like:

• to adopt → adoption
• to decide → decision

الـ is the Arabic definite article, usually translated as the.

But Arabic uses the definite article more often than English does, especially with abstract ideas and general concepts. So التبني can mean:

• adoption
• or more literally the adoption

In English, we usually say Adoption is important, without the. In Arabic, using الـ here is very natural.

Because مهم is the predicate adjective in a nominal sentence, and in this kind of sentence it is normally indefinite.

So:

• التبني مهم = Adoption is important

But:

• التبني المهم would mean something like the important adoption

That second one is not a full sentence. It is just a noun phrase.

So this is an important contrast:

• التبني مهم = a sentence
• التبني المهم = a phrase

Yes, adjectives do agree with the noun they describe or the subject they refer to. Here, التبني is grammatically masculine, so the adjective is also masculine:

• التبني مهم

If the noun were feminine, you would use مهمة instead.

For example:

• القضية مهمة = The issue is important

So مهم is correct because التبني is treated as masculine.

This is the sun letter rule.

The word begins with:

• ال + تبني

Because ت is a sun letter, the ل in الـ is not pronounced. Instead, the ت is emphasized/doubled in pronunciation.

So:

• written: التبني
• pronounced roughly: at-tabannī / it-tabannī / et-tabannī

This is why learners often hear a difference between spelling and pronunciation.

The written sentence التبني مهم works perfectly well as Standard Arabic, and it is also completely understandable in Levantine.

What changes more in Levantine is the pronunciation, not the basic written form.

So you can think of it like this:

• Writing: very close to Standard Arabic
• Speech: more Levantine pronunciation, like it-tabannī mhimm

Here, the final ي is part of the noun itself and gives the ending a long -ī sound.

So التبني is pronounced roughly tabannī.

It is not the possessive my here.
The whole word means adoption, not my adoption.

In unvoweled Arabic writing, endings can sometimes look confusing to learners, but in this sentence the meaning is clearly adoption.

Yes. Even though everyday Arabic writing often leaves out the shadda mark, the pronunciation includes doubled consonants.

Two important ones here are:

• التبنّي → the n is doubled: tabannī
• مهمّ → the final m is doubled: muhimm / mhimm

So if you pronounce everything too lightly, it may sound less natural.

More or less, yes:

• التبني = adoption
• مهم = important

So the literal structure is:

• Adoption important

But natural English requires is, so the correct translation is:

• Adoption is important.

This is a very common difference between Arabic and English sentence structure.