Learn Portuguese (Portugal): O cão está debaixo da árvore.

The basic phrase for "under" is debaixo de. However, in Portuguese, de almost always squishes together with the word for "the". Since árvore (tree) is feminine, "the tree" is a árvore. When you combine de + a, you get da. So, debaixo da effectively means "under the".

Why do we use está here instead of é for "is"?

We use está (from the verb estar) because it describes where something is located. Even though you might have learned that estar is for temporary states and ser is for permanent things, physical location is a specific rule. For showing where something is, we always use estar.

I noticed árvore ends in "e". How do we know it is feminine?

Words ending in "e" can be either masculine or feminine in Portuguese, so you just have to memorize their gender as you learn them! The fact that we use da (which contains the feminine "the", a) right before it is a perfect clue in this sentence that árvore is feminine.

Why does árvore have an accent mark?

The acute accent on the "á" tells you to put your spoken stress on that very first syllable (AR-vo-re). Without it, the natural rules of Portuguese pronunciation would make you stress the middle syllable.