Learn Portuguese (Portugal): Há uma árvore no meio do quintal. - There is a tree in the middle of the backyard.

In Portugal, you should always use Há (from the verb haver) to mean "there is" or "there are" when talking about existence. You might hear Brazilians use Tem (from ter, meaning "to have") for this, but in European Portuguese, ter is only used for possession (like "I have a car").

If there were two trees in the backyard, would Há change to plural?

No, it stays exactly the same! When meaning "there is" or "there are", the verb haver is impersonal, meaning it only ever exists in the third-person singular form. So you would say Há duas árvores (There are two trees).

Why does the sentence use no before meio and do before quintal?

The phrase for "in the middle of" is no meio de. The no is a contraction of em (in) + o (the masculine "the" for meio). Then, you need another contraction for the second part: de (of) combines with o (the masculine "the" for quintal) to make do.

The word árvore ends in an "e". Why does it use the feminine article uma?