Learn German: Das Baby weinte, darum tröstete der große Bruder es. - The baby was crying, so the big brother comforted it.

weinte is the simple past (Präteritum) of the regular verb weinen (to cry). To form the simple past of regular (weak) verbs, you take the verb stem (wein-) and add -te for the 3rd person singular (er/sie/es) form.

Why does tröstete have an extra 'e' in the middle compared to weinte?

The verb trösten (to comfort) has a stem that ends in a 't' (tröst-). If we just added the past tense ending -te, it would be tröstte, which is hard to pronounce. To fix this, German inserts an extra e between the stem and the ending, making it tröstete.

Why does the verb tröstete come right after darum? Shouldn't the subject come first?

darum (therefore / so) is an adverb. When you put an adverb in the first position of a clause, the verb must stay in the second position (the V2 rule). This pushes the subject (der große Bruder) into the third position, right after the verb.

Why is the pronoun at the end es?