Learn Spanish (Spain): Ayer compré esas fresas rojas en el mercado. - Yesterday I bought those red strawberries in the market.

The word ayer (yesterday) gives us a big clue. You use the preterite tense (compré) for actions that started and finished at a specific time in the past. Since buying the strawberries was a single, completed event yesterday, the preterite is the right choice.

Why do we use esas instead of estas or aquellas?

Esas means 'those' for things that are a medium distance away. It has to match the feminine, plural word fresas. If the strawberries were right here in your hands, you would use estas (these), and if they were far away from both of you, you would use aquellas (those over there).

Why does the color come after the word for strawberries (fresas rojas)?

In Spanish, descriptive words like colors usually come after the noun they describe. Also, notice how rojas ends in '-as' to perfectly match the feminine plural word fresas.

Could I say al mercado instead of en el mercado?

Not in this sentence. En el mercado means 'in the market' or 'at the market,' which tells us where the action of buying happened. You would use al mercado (to the market) if you were talking about movement towards a place, like saying you went to the market.