Learn Portuguese (Portugal): Eu tenho um livro e uma caneta.

In Portuguese, words for 'a' or 'an' (indefinite articles) must match the gender of the noun they describe. Because livro (book) is a masculine word, it takes the masculine article um. Caneta (pen) is feminine, so it takes the feminine article uma.

Can um and uma also mean the number one?

Yes! In Portuguese, the words for 'a/an' and the number 'one' are exactly the same. Um livro can mean 'a book' or 'one book' depending on the context.

Is it mandatory to use Eu in this sentence?

No, you could just say Tenho um livro e uma caneta. Since the verb form tenho is only ever used with eu (I), the pronoun is often dropped in everyday Portuguese unless you want to add emphasis.

Why does the verb tenho look so different from its dictionary form?

The dictionary form of this verb is ter (to have). It is a highly irregular verb, meaning it does not follow the normal ending rules. Tenho is the specific form for 'I have', which is extremely common and needs to be memorized as a set word early on.