Learn Portuguese (Portugal): Há ali um cão grande.

um is the masculine singular word for "a" or "an". We use it here because cão (dog) is a masculine noun. If we were talking about a feminine noun, like casa (house), we would use uma.

What does Há mean in this sentence?

Há means "there is" or "there are". It comes from the verb haver. It is a very handy word for beginners because it stays exactly the same whether you are talking about one dog or five dogs.

Why is the word order cão grande instead of grande cão?

In English, we put the describing word (adjective) before the noun, like "big dog". In Portuguese, the describing word usually comes directly after the noun, so it literally translates to "dog big" (cão grande).

Since cão is masculine, shouldn't grande end in an "o"?

Not all adjectives change their ending to match masculine or feminine words. Adjectives that end in "-e", like grande (big), stay the same for both. You would say um cão grande (masculine) and also uma casa grande (feminine).