Learn Portuguese (Portugal): Há uma luz no teto. - There is a light on the ceiling.

Há comes from the verb haver and means "there is" or "there are" when talking about existence. It is an impersonal verb here, which means it never changes form to become plural. To say "there are lights", you still use the exact same word: Há luzes no teto.

Is the word luz feminine even though it doesn't end in "-a"?

Yes, luz is a feminine noun, which is why it takes the feminine article uma. While most feminine words end in "-a", nouns ending in "-z" can be either masculine (like o nariz, the nose) or feminine (like a luz, the light). You just have to memorize the gender for these specific words.

Why do we use no for "on the"? Shouldn't "on" be a different word?

In English, we make a strict difference between being "in", "on", or "at" a location. Portuguese simplifies this by using the preposition em for all three. Because teto (ceiling) is a masculine noun, we combine em + o (the) to make the mandatory contraction no.

Does teto mean both ceiling and roof?

No, teto specifically refers to the ceiling (the inside upper surface of a room). If you want to talk about the roof on the outside of a house, you need to use a completely different word: telhado.