Learn Spanish (Spain): Mi sobrina creció mucho el año pasado.

Crecer (to grow) is an -er verb. To form the past tense (preterite) for "he" or "she" (in this case, my niece), you take off the -er and add the regular ending -ió.

Wait, isn't crecer an irregular verb? Why does it have a regular ending here?

You might be thinking of its present tense "I" form, which is irregular (yo crezco). However, in the preterite tense, crecer is completely regular and follows the exact same pattern as other regular -er verbs.

Since sobrina (niece) is feminine, why is it mucho and not mucha?

Here, mucho means "a lot" and it's describing the action word creció (how much she grew). When it describes an action, it is an adverb, and adverbs never change their endings to match gender or number. You would only use mucha if it were describing a feminine noun (like mucha paciencia, "a lot of patience").

Why do we use the preterite tense here instead of the imperfect tense?

The phrase el año pasado (last year) is the key. It gives us a specific, finished block of time in the past. When an action is viewed as a completed event within a defined timeframe, Spanish uses the preterite.