Learn Turkish: Dikkatli ol, yoksa zehirli yılan ısırır. - Be careful, otherwise the poisonous snake will bite.

It literally comes from yok (non-existent) plus the conditional suffix -sa (if). But when used as a connecting word between two sentences like this, it translates to 'otherwise' or 'or else'. It introduces the consequence of what happens if you don't follow the command to be careful.

Why does the English translation say 'will bite' but the Turkish verb ısırır isn't in the future tense?

The verb ısırır is in the aorist (simple present) tense, which you can see from the -ır ending. Turkish often uses this tense for general facts, natural behaviors, and predictable consequences. Since biting is what a snake naturally does if you aren't careful, the aorist fits perfectly here instead of the future tense ısıracak.

Both dikkatli and zehirli end in -li. Is this a coincidence?

Not at all! The suffix -li turns nouns into adjectives meaning 'having' or 'containing' that noun. Dikkat (attention or care) becomes dikkatli (careful). Zehir (poison) becomes zehirli (poisonous).

Why is there no word for 'the' or 'a' before zehirli yılan?