Learn Russian: Грузчики каждый день таскают тяжести, поэтому они такие сильные. - Movers haul heavy loads every day, so they are so strong.

Таскают is the multidirectional verb of motion for dragging or hauling. Because the sentence describes a habitual, repeated action (каждый день) rather than a single trip in one direction, you must use the multidirectional таскать instead of the unidirectional тащить.

What is the root of the word грузчики?

It comes from the noun груз, which means cargo, load, or freight. The suffix -чик is used to form a noun representing a person's profession or role, making a грузчик literally a loader or a mover.

Why does the sentence use такие сильные instead of так сильные for so strong?

In Russian, так is used to modify verbs or adverbs, while такой and its forms modify nouns and adjectives. Even though English says so strong, Russian pairs the plural modifier такие with the plural adjective сильные.

Is тяжести a plural or singular noun here?

It is in the plural. The singular is тяжесть (heaviness or a heavy object), which is a feminine noun ending in a soft sign. To form the accusative plural of these feminine soft-sign nouns, the soft sign changes to -и, giving us тяжести.