Learn Russian: Этот человек рано завтракал.

The dictionary form (infinitive) of this verb is завтракать (to have breakfast). Because it is a regular verb, you simply drop the -ть at the end and replace it with the masculine past tense marker -л.

Since a person can be male or female, why do we use the masculine past tense ending -л?

The Russian word человек (person) is always grammatically masculine. Because of this, any connected words must also be masculine. That is why we use the masculine word for "this" (Этот) and the masculine past tense verb ending (-л), even if the person you are talking about happens to be a woman.

Does Russian have a separate word for "had" in "had breakfast"?

No. In English, we often use a combination of words like "had breakfast" or "ate breakfast". In Russian, завтракать is a single verb that means "to eat breakfast". So, the one word завтракал covers the whole phrase "had breakfast".

Can I put рано at the end of the sentence like we do with "early" in English?

You can, and people will understand you, but it sounds slightly less natural. In Russian, adverbs like рано (early) usually go right before the verb they describe.