Learn Portuguese (Portugal): Eu como o pequeno-almoço em casa.

Comer (to eat) is a regular verb ending in -er. To make the eu (I) form, you drop the -er from the end of the verb and add -o. This gives us como.

Why do we need the word o before pequeno-almoço?

In Portuguese, you generally use definite articles (words for 'the', like o or a) before the names of meals. While in English we say 'I eat breakfast', in Portuguese you literally say 'I eat the breakfast'.

What does pequeno-almoço literally translate to?

It literally means 'small lunch'. Pequeno means 'small' and almoço means 'lunch'. Notice that the adjective (pequeno) comes before the noun here, which is a common exception for a few specific words, even though Portuguese usually puts the adjective after the noun.

Why does the sentence use em casa instead of na casa?

Em casa is a fixed phrase that means 'at home'. You just use the preposition em (in/at) without an article. If you said na casa (a combination of em + a), it would mean 'in the house', and you would usually need to specify whose house.