"I have a problem."
Translation:Yo tengo un problema.
Because problema, even though it ends with an A, is masculine.
While most nouns ending in "-a" are feminine, there are exceptions. Some — including many that end in "-ma" — derive from Greek words and are masculine nouns despite ending in "-a". There are no hard-and-fast rules for these, so you'll just need to make note of them when you come across them.
June 16, 2018 Here's a discussion of Spanish gender exceptions: https://www.lawlessspanish.com/grammar/nouns/gender-exceptions/
Some are shortened versions of feminine words (such as foto/fotografia). There are also some -o words that are feminine (la mano).
Note that many of these exceptions end in -ma, -pa, & -te. When I encounter an unfamiliar word with those endings, I check the gender in the dictionary.
There really aren't that many, and you'll remember the ones you use a lot.
El día is indeed masculine.
However, just to confuse the issue, the el in el agua is there just because it's hard to say la agua. Agua is feminine for all other uses. In fact, you would say, El agua es preciosA. http://www.spanishdict.com/translate/the%20water
This is why when you run across these odd words, you need to check them in the dictionary if you're the least bit uncertain regarding the gender. It's taken a long time for me to get agua correct consistently. July 10, 2018
Duo is feeling self assertive today, so has to add the yo.
As you well know, the yo is optional. Be sure to report it if DL counts it wrong to leave it off. Remember to check for other errors, too. July 20, 2018
It really surprised me that "problema" was masculine rather than feminine but I now know for next time!
Answer -"You used the wrong word. tengo un asunto." Asunto ???? Asunto has not been on Duolingo before.
Pretty much. The O is masculine, A is feminine rule is pretty solid, but it has its exceptions that you just need to memorize. Off the top of my head I can think of: El problema El tema El idioma La mano