Learn Portuguese (Portugal): O caixa pôs o dinheiro dentro da caixa. - The cashier put the money inside the box.

This is a dual-gender noun where the definite article completely changes the meaning. As a masculine noun (o caixa), it refers to a male cashier or bank teller. As a feminine noun (a caixa), it means the physical object: the box, carton, or cash register. Note that a female cashier is also called a caixa — in that case, you just have to rely on context to know if you're talking about a woman or a cardboard box!

Is pôs the past tense of pôr or poder?

It comes from pôr (to put). Pôs is the third-person singular in the simple past (pretérito perfeito), meaning he put. It is very easy to confuse with forms of poder (to be able to), especially since they look and sound similar. However, the equivalent third-person past tense for poder is pôde (he could).

Could we just say na caixa instead of dentro da caixa?

Yes, you could say pôs o dinheiro na caixa (put the money in the box), which is perfectly correct and very common. However, using dentro da (inside of the) emphasizes the physical interior of the container, making the action sound slightly more precise.