An adverb of frequency goes before a main verb
(except with To Be)

subject
+
adverb
+
main verb

I always run in the park

He sometimes studies in the evening

An adverb of frecuency goes after the verb “To Be”

subject
+
to be
+
adverb

You are usually very busy at 2:00 p.m.

She is never very sleepy at night

We can also use the following adverbs at the start of a sentence:

Usually, normally, often, sometimes

Often, I like to eat Thai food

BUT we cannot use the following at the beginning of a sentence:

Always, hardly, never