The ordinal numbers in Spanish have masculine and feminine forms, differing in the -o and -a endings. A trick spot regards primero and tercero, which each drop the final -o before a masculine singular noun. For example: El primer año. (The first year.)

The infinative of a verb is the unconjugated form that appears in the vocabulary. It generally translates as to ..., for example:
progresar = to progress. There are many constructions in Spanish that use the infinative form of the verb.

The personal a has no equivalent in English. It is used before a direct object that refers to a specific person. It is not used if the direct object is not a person. It is also not used with the verb tener, which you will meet in the next lesson. The personal a is subject to the contraction discussed earlier.

Possessive adjectives preceed the nouns they refer to, mi libro rojo. They also agree in number (and gender in the case of nuestro and vuestro), with the noun they refer to, mis libros rojos. This is important, they agree in number not with the owner, but with what is owned.

The possessive adjectives su and sus, have several possible meanings. Su libro could mean any of his, hers, your (Ud.), its, or their book. When the meaning is unclear you can instead use the de él, de Ud., etc. form.
el libro de él (His book)
el libro de Uds. (Your (form. pl.) book)