Learn Portuguese (Portugal): Tu sabes que a notícia é triste.

Yes! While sabes (you know), sabe (he/she/it knows), and sabemos (we know) follow the regular -er verb pattern, the eu (I) form is highly irregular. Instead of "sabo", it is sei.

Why use sabes here instead of conheces? Don't they both mean "to know"?

Yes, but they are used differently. Saber is used for knowing facts, information, or how to do something. Here, you know a fact: that the news is sad. You use conhecer when you are familiar with a person, place, or thing (like knowing a friend or a city).

In English we say "the news", which ends in an 's'. Why is a notícia singular here?

In Portuguese, uma notícia refers to a single news story or piece of news. If you want to talk about "the news" as a general concept or broadcast, you would use the plural as notícias.

Since a notícia is feminine, shouldn't the adjective be "trista"?