Adding an answer simply because the one in 2018 was rather short.
Where adverbs are placed in English sentences can be highly flexible and often (but not always) a matter of taste. There is some rough 'general guidance' to help you. But they're not strict rules, and a lot just depends on familiarity with the language.
A reasonable summary for learner can he found at ThoughtCo. However, it gives the impression of more prescriptive rules than is the case in reality.
The points made there can be summarized as:
Often/usually placed at start of a sentence or clause to join it to a previous sentence or clause.
It has been raining all day. However, the sun just came out.
It has been raining all day. The sun, however, just came out.
It has been raining all day. The sun just came out, however.
are both acceptable, if uncommon and perhaps more likely in written than spoken English.
Mostly the aliens come at night.
The aliens mostly come at night.
The aliens come at night mostly.
But compare "always". For reasons that don't make much sense, you can't easily use it in the same way as "mostly" or "sometimes".
I always procrastinate at work.
is fine, but:
Always I procrastinate at work
immediately sounds like a person speaking English as a second language. It's perfectly understandable, but it sounds foreign.
Adverbs that add focus or emphasis, generally are more likely to appear in the 'mid position' in sentences, with the adverb appearing immediately before the verb is modifies.
I'll certainly buy the a copy of his book.
She often forgets her umbrella.
Adverbs of Manner, Place and Time
All tend to be placed at the end of sentences or clauses.
Barbara is cooking pasta downstairs.
I am leaving tomorrow.
It's probably worth checking a dictionary if in doubt to get a sense of how they are used in practice, as for every example given here it would be easy to give examples that do something different.