Learn Spanish (Latin America): Fue la grasa lo que hizo el daño.

This lesson is about using puro for emphatic exclusivity. How could we use it in this sentence to say "It was nothing but fat that did the damage"?

You could say Fue pura grasa lo que hizo el daño. In Latin America, placing puro (or pura here, to match the feminine noun grasa) before a noun is a very common way to emphasize "nothing but," "only," or "purely."

Why does this sentence start with Fue?

This is a cleft sentence, which is a structure used to emphasize a specific part of a statement. Just like in English ("It was the fat that..."), Spanish uses the verb ser (here in the preterite as fue) followed by the emphasized element (la grasa), and then a relative pronoun (lo que) to connect the rest of the information.

Why do we use lo que here instead of just que for "that"?

In cleft sentences that emphasize a noun, you will often see lo que (or el que, la que, etc.) used to mean "the thing that" or "that which." While English just says "It was the fat that...", Spanish conceptually phrases it as "It was the fat, the thing that did the damage."

Is hacer daño a set phrase? Why is there an el in hizo el daño?