Thank you so much! It's very useful.

In English the correct phrase would be "most often" or "mostly". Mostly often is not used in English.  Most often and mostly both mean the same thing and can be used in place of each other a lot. For instance, you can say "I mostly walk to school" or "most often, I walk to school." Note that most often goes in front of the pronoun "I" and mostly comes after. Mostly can also go to the end of the sentence, making it "I walk to school mostly." and most often can come after I, making it "I most often walk to school". These are all acceptable ways to answer in English. Hope this helps.