Learn Danish: Lejligheden er til leje fra marts. - The apartment is for rent from March.

In Danish, the preposition til (which usually means "to") is frequently used to show the state, purpose, or availability of something. Just as til salg translates to "for sale," til leje translates to "for rent." Prepositions rarely translate one-to-one between languages, so it is best to memorize this as a fixed phrase.

Is leje a noun or a verb in this sentence?

It is a noun here, meaning "rent" or "hire." However, it looks exactly the same as the infinitive verb at leje (to rent), which you will also see very often. Because it directly follows the preposition til in this fixed expression, it is functioning as a noun.

Why isn't the month marts capitalized?

Unlike in English, Danish does not capitalize the names of months, days of the week, nationalities, or languages. They are always written in lowercase unless they happen to be the very first word of a sentence.

How do I know the gender of lejlighed to make it lejligheden?

The word lejlighed (apartment) ends with the common noun suffix -hed. In Danish, absolutely all nouns ending in -hed are common gender (en-words). This is a helpful rule to remember, as it means they will always take the -en ending when you want to say "the".