Questions & Answers about Mezcla el tomate y la cebolla.
Why does the verb mezclar (to mix) end in an 'a' here instead of 'as' for tú?
Because this is an affirmative command (telling someone to do something)! For regular -ar verbs, the tú command uses the same ending as the él/ella/usted present tense form. You basically just drop the 's' from mezclas, leaving you with mezcla.
Doesn't mezcla also mean 'he/she mixes'? How can I tell this is a command?
You're exactly right! By itself, mezcla could mean 'he mixes' or 'she mixes'. But here, there is no subject pronoun like él or ella in front of it. Because it reads like a direct instruction—like in a recipe—the context tells us it's a command telling you to mix.
If I were giving this instruction to someone I address as usted (formal), how would the verb change?
In English, a recipe might just say 'Mix tomato and onion' without 'the'. Do I really need el and la here?