Learn Portuguese (Portugal): Eu tenho vinte anos.

In English we say how old we are, but in Portuguese you say how many years you have. This is why we use tenho, the "I" form of the verb ter (to have). Literally, this sentence means "I have twenty years."

Since ter is an irregular verb, how does it change to become tenho?

Tenho is the first-person singular (Eu) form of ter. It is highly irregular because the base of the word changes entirely from ter to tenh-. Because it doesn't follow normal ending rules, you simply have to memorize that eu tenho means "I have."

Can I drop the word Eu and just say Tenho vinte anos?

Yes, absolutely! Because the verb form tenho is only ever used for "I" (Eu), the pronoun is optional. Dropping the pronoun is actually the most common and natural way to speak in Portuguese.

Will I use ter for other things where English uses "to be"?

Yes! Just like expressing your age, Portuguese uses ter for many physical feelings or states. For example, you will also use it to say you are hungry (ter fome, literally "to have hunger") or thirsty (ter sede, "to have thirst").