This is the 157th most frequent German word.
monaten
"Monaten" means "months" (dative or plural form) in English.
The word "Monaten" is in dative plural form here, used with the preposition "seit" to indicate a period of time.
Ich habe seit drei Monaten Deutsch gelernt.
I have been learning German for three months.
Here, "Monaten" is again in dative plural, this time used with the preposition "vor" to indicate a time earlier than now.
Vor zwei Monaten bin ich nach Berlin gezogen.
I moved to Berlin two months ago.
In this example, "Monaten" is still in dative plural, used with the preposition "in" to talk about a future point in time.