Learn Portuguese (Portugal): Encontrei a chave ontem.

In Portuguese, you often don't need to use subject pronouns like eu (I) because the verb ending already tells you who did the action. The verb here is encontrar (to find). In the simple past tense, the -ei ending always means "I" did it.

Is encontrei formed just like other regular -ar verbs in the past tense?

Exactly! It follows the pattern you are learning for regular verbs ending in -ar. To talk about a completed action in the past, you take off the -ar and add -ei for the "I" form. For example, falar (to speak) becomes falei (I spoke), and encontrar becomes encontrei.

Does the "ch" in chave sound like the "ch" in the English word "cheese"?

No, in European Portuguese, the letters ch make a soft "sh" sound, like in the English word "shoe". Therefore, chave is pronounced more like "shahv".

Can I put ontem at the beginning of the sentence instead?

Yes, you could say Ontem encontrei a chave. Both word orders are perfectly correct and natural in Portuguese. Putting ontem at the beginning simply places a little more focus on the fact that it happened yesterday.