Learn Arabic (Modern Standard): يناير أول شهر في السنة.

In Arabic, when talking about units of time belonging to a larger period, you typically use في (in). So, instead of saying "the first month of the year," Arabic literally says "the first month in the year" (في السنة).

Why isn't there a word for "the" (ال) before "first month"?

To say "the first [noun]" in Arabic, you use a special structure: the word أول (first) followed immediately by a singular noun without ال (the). Together, أول شهر forms an Idafa (a relationship or possession phrase) that means "the first month," even without the word "the" explicitly written.

Is يناير the only way to say "January" in Arabic?

No, Arabic actually has a few sets of month names! يناير is used in Egypt, the Gulf, and much of North Africa, and is universally understood in Modern Standard Arabic. However, in the Levant (like Lebanon, Jordan, and Syria) and Iraq, January is called كانون الثاني. It is useful to recognize both.

If I am pronouncing the formal grammatical endings (tashkeel), what is the ending on شهر?

It takes the "in" sound (kasratayn), making it shahrin. This is because it is the second word in an Idafa phrase, which forces it into the genitive case. In everyday spoken Arabic, however, you usually just drop the ending and say shahr.