Learn Arabic (Modern Standard): السنة اثنا عشر شهرا.

For numbers 11 through 99, Arabic always uses a singular noun. This noun must also be in the accusative case, which is why it ends with the '-an' sound. So instead of saying 'twelve months', Arabic literally says 'twelve month'.

How do we know to use the masculine form اثنا عشر for the number twelve?

For the numbers 11 and 12, both parts of the number must match the gender of the noun being counted. Since the word for month, شهر, is masculine, both parts of the number 12 take their masculine forms (اثنا and عشر).

Why is there an extra alif at the end of شهرا?

That extra alif is added to carry the double fatha symbol (tanwin fath), which makes the '-an' sound for the accusative case. Most words that don't end in a taa marbuta need this extra alif when they are in this case.

Why isn't there a word for 'is' between السنة and اثنا?

In Arabic, equational sentences (where A is B) do not use a 'to be' verb in the present tense. You simply put the subject (السنة) and the predicate (اثنا عشر شهرا) side by side to mean 'The year is twelve months'.

Why is the first part of the number written as اثنا and not اثني?