Primero and tercero become primer and tercer before a noun (Spanish apócope)

Shortened forms of primero and tercero in Spanish

Some ordinal numbers (i.e first, second, third...) in Spanish have a shorter form when they are placed before a singular masculine noun. This affects two ordinal numbers:

• Primer (first)
• Tercer (third)

Have a look and listen to the following examples:

Feminine nouns

The shorter form is not used in front of feminine nouns:

Plurals

The short form is not used when accompanying feminine or masculine plural nouns either:

Pronoun Usage

The long form of these adjectives (primero / tercero) are used as pronouns (i.e. not accompanying a noun) they do not change and retain their long form.

Have a look:

For a list of ordinal numbers in Spanish see  Ordinal adjectives from 1st to 20th in Spanish

To see other cases of apocopation see

• Bueno, malo, grande become buen, mal, gran before a noun (apócope) 
• Using cualquier/a to express "any" in Spanish (indefinite adjectives)