Learn Spanish (Latin America): Cuando la mudanza termine, vamos a hacer una gran fiesta. - When the move finishes, we are going to throw a big party.

Because the move hasn't finished yet! When we use cuando (when) to talk about an action that will happen in the future, Spanish requires the subjunctive mood. Since terminar is an -ar verb, its form in the present subjunctive changes the -a to an -e, giving us termine.

Is la mudanza related to the verb mudarse (to move residences)?

Yes! Mudarse means to move from one house to another. By replacing the verb ending with the suffix -nza, it becomes the noun la mudanza (the move).

Why does it say una gran fiesta instead of una fiesta grande?

When the adjective grande comes before any singular noun (masculine or feminine), it drops the final -de and becomes gran. You can also say una fiesta grande, but placing gran before the noun often makes it sound more emphatic, like a grand or spectacular party.

Does hacer mean 'to throw' here?

Literally, hacer means to do or to make. In English you throw a party, but in Spanish, you make one. Hacer una fiesta is the standard everyday way to say 'to throw a party' or 'to have a party'.