Learn Portuguese (Portugal): O rapaz cortou-se e viu o sangue.

This is the reflexive verb cortar-se (to cut oneself) in the past tense. For the ele/ela/você form, cortar becomes cortou. In European Portuguese, pronouns like the reflexive se generally attach to the end of the verb with a hyphen in positive sentences.

Why do we use cortou-se instead of cortava-se here?

We use the Pretérito Perfeito Simples (cortou-se) because the boy cutting himself was a single, completed, and sudden event in the past. If we used the Pretérito Imperfeito (cortava-se), it would mean he used to cut himself repeatedly as a habit.

What is the infinitive of the verb viu?

viu is the past tense (Pretérito Perfeito Simples) of the verb ver (to see) for the ele/ela/você form. ver is an irregular verb, which is why it changes completely from its infinitive stem in the past tense.

How do I know if words like sangue are masculine or feminine?

Since sangue ends in -e, its gender is not immediately obvious like words ending in -o or -a, so you just have to memorize it! In this sentence, you can easily spot that it is masculine because it is paired with the masculine definite article o.