In Gaelic there are 8 different forms of the definite article. The following 6 are most common;

An

Am

A’

An t-

Na

Na h-

The definite article An is used in the following instances

Feminine words beginning with a vowel

Masculine words beginning with c, g & s

M&F words beginning D, L, N, R & T

F words beginning with S except si, sn, sr, or s+vowel

F words beginning with F+vowel

Feminine words like this also requirecthe letter h to be inserted before the vowel when using the definite article

The alphabet

Aibidil (f)

Face - aghaidh (f)

The church

Eaglais (f)