Learn Portuguese (Portugal): Há muito trânsito no centro. - There is a lot of traffic in the center.

muito is acting as a quantifier here (meaning "a lot of"), so it must match the gender and number of the noun it describes. Since trânsito (traffic) is a masculine, singular word, we use the masculine, singular form: muito.

What is Há and how does it work?

Há means "there is" or "there are". It comes from the verb haver. The great thing about Há is that it never changes form. Whether you are talking about one thing or many things (like a lot of traffic), it is always just Há.

Why is it no centro instead of em centro?

To say "in the" or "at the", Portuguese combines the preposition em (in/on/at) with the word for "the". Since centro is a masculine word, we combine em + o (the) to make the mandatory contraction no.

What does the accent mark on trânsito do?

The circumflex accent (the "little hat") on the â tells you to put the spoken stress on that first syllable. It also means the vowel has a more closed, slightly nasal sound, rather than a wide open "ah" sound.