Learn Spanish (Spain): Subimos andando al quinto piso.

English usually uses a verb of movement (walk, run) plus a direction word (up, down). Spanish does the exact opposite! It uses a verb of direction (subir = to go up) followed by a gerund (andando = walking) to show how the movement was done. Literally, this says "We went up walking."

Is subimos in the present tense or the past tense?

It is in the preterite (past) tense here, meaning "we went up". For regular -ir verbs like subir, the nosotros (we) form is spelled and pronounced exactly the same in both the present and the preterite past. You have to rely on context to know if it means "we go up" or "we went up."

Why does quinto come before the noun instead of after it?

While most descriptive adjectives in Spanish come after the noun, ordinal numbers (numbers that show order, like first, second, fifth) are placed before the noun. Just remember that they still need to match the noun's gender and number, which is why it is quinto to match the masculine singular noun piso.

I learned that piso means "apartment" or "flat". Does it also mean "floor"?

Yes, especially in Spain. Piso is widely used to mean an apartment, but its original meaning is the floor or story of a building. In this sentence, going up to the quinto piso means going up to the fifth floor.