Learn Spanish (Latin America): Todavía no hemos escrito en las cajas.

In Spanish, todavía generally means "still". To say "not yet", you must pair it with no to make todavía no. It is frequently used with the present perfect to show that an action hasn't happened up to this point in time, but it probably will later.

Can I put todavía no at the end of the sentence like we do with "yet" in English?

Yes! You can also say No hemos escrito en las cajas todavía. Both options are perfectly natural in Spanish. Putting todavía no at the beginning just adds a little extra focus to the fact that the action hasn't happened yet.

Why is the word for "written" escrito instead of ending in "-ido"?

The verb escribir (to write) has an irregular past participle. Instead of following the regular "-ir" verb rules (which would make it "escribido", a word that doesn't exist!), it becomes escrito. This is a very common irregular form you'll just need to memorize.

The English translation says "on the boxes," but doesn't en mean "in"?

En is a very flexible preposition that can translate to "in", "on", or "at" depending on the context. Since you write on the surface of a box, en las cajas is the standard way to express "on the boxes" in Spanish.