Learn Turkish: Çadırı kapat, yoksa böcekler ve yılanlar çadıra girer. - Close the tent, otherwise insects and snakes will enter the tent.

Yes, they are very different! While veya just means 'or' for making a choice between things, yoksa is used for consequences. It literally comes from yok (there is not) and -sa (if), so it means 'if not'. In this sentence, it acts exactly like 'otherwise' or 'or else' in English, connecting the command to the bad thing that will happen if you do not do it.

Why does the word for tent appear as both çadırı and çadıra?

The suffix changes based on the word's role in the sentence. In çadırı kapat, it takes the accusative case suffix -ı because the tent is the specific direct object you are closing. In çadıra girer, it takes the dative case suffix -a because it shows the direction or destination the animals are moving towards, meaning 'into the tent'.

The English translation says 'will enter', so why does the verb girer use the aorist (simple present) tense instead of the future tense?

You could technically use the future tense girecek, but the aorist tense (ending in -er) is very common for expressing natural consequences and general truths. Because bugs entering an open tent is a predictable rule rather than a uniquely scheduled event, the aorist sounds completely natural here.

Since the subject is multiple insects and snakes, shouldn't the verb be plural like girerler?