Learn Portuguese (Portugal): Procurei o livro ontem. - I looked for the book yesterday.

In the simple past tense, regular verbs ending in -ar (like procurar) drop the -ar and add -ei for the "I" (eu) form. Because the ending -ei already tells us who did the action, we can usually leave out the word eu. If it were "he looked", the ending would be different: ele procurou.

The English translation says "looked for", but where is the word for "for" in the Portuguese sentence?

The Portuguese verb procurar inherently means "to look for" or "to search for". You don't need to add an extra preposition like "for" after it. You just put the thing you are looking for directly after the verb, as in procurei o livro.

If I wanted to say "He looked for the book yesterday", how would the verb change?

Instead of the -ei ending for eu (I), you would use the -ou ending for ele (he). The sentence would become Ele procurou o livro ontem. For regular -ar verbs in the past tense, the other key endings are -aste for tu (you, informal), -ámos for nós (we), and -aram for eles / elas / vocês (they / you all).

Could I put ontem at the beginning of the sentence instead of the end?