Learn Romanian: Îl folosesc luni și marți. - I use it on Monday and Tuesday.

In Romanian, the subject eu (I) is usually left out because the verb folosesc (I use) already tells us who is doing the action. Îl is a short word meaning "it" (referring to a masculine or neuter noun), and in Romanian, these short object words are placed directly before the verb. So literally, the sentence reads as "It (I) use".

The English translation says "on Monday and Tuesday", but there is no word for "on" in the Romanian sentence. Why?

In Romanian, you do not need to add a preposition like "on" before a day of the week to say that an action happens on that day. Simply using the word luni automatically means "on Monday" in this context.

Why aren't luni and marți capitalized?

Unlike in English, days of the week in Romanian are always written with lowercase letters, unless they happen to be the very first word of a sentence.

Do luni and marți end in "i" because they are plural words?

No, the i at the end of luni (Monday) and marți (Tuesday) is just a normal part of their singular spelling. As you learn the rest of the week, you will notice that almost all the days end in an "i".