"لا أَعْرِف اِسْمهُ."

Translation: I do not know his name.

[koppelshira]

The vocal does not pronounce the "u" at the end of words.

[BillDe]

Isn't إسمة feminine?

[CarstenLu]

No, it is اسم (name) with the possessive suffix 3rd person singular masculine ـه (note that it is lacking the two dots).

[GabrielBou109736]

Why isn't Ism supported by a hamza with the kasra? Like 2a3rif with the fatha?

[CarstenLu]

Because اسم does not start with hamza (أ or إ) but with wasla ٱ.

As a result, there is no guttural stop at the beginning of اسم but you contract it with the previous word. If the previous word ends with a (short or long) vowel, the initial ا even becomes silent, e.g., باسم الله bi-smi-llah in Allah's name (actually contains two silent alifs).

[GabrielBou109736]

Thanks! It's particular to ism then? And the sentence is pronounced a such: lā 2a3rif-ismu?

[CarstenLu]

There are other words than اسم which start with wasla and not with hamza. The most prominent example is the definite article الـ. So, yes, it is particular to the word but not only to اسم.

A hamza should be always written (otherwise, it is a spelling error, as far as I understand).
A wasla is usually omitted so that you see a bare alif at the beginning of a word.

As to the pronunciation of this sentence, yes, you hear lA 2a3rif-ismuh in dialects. In MSA, however, it would be lA 2a3rif-u-smuh.

[avisarid1]

Isn't it ismuhu? I think we heard it like that in other excercises.
Also, in that case, why does the feminine version
اسمها
Have alif at the end and the masculine does not have waw?