Frequency adverbs tell HOW OFTEN something happens.
always 100% (Siempre)
nearly/almost always 90% (Casi siempre)
usually 80% (Normalmente)
very often/frequently 70% (Frecuentemente)
often 60% (a menudo)
sometimes 50% (a veces)
occasionally 40% (en ocasiones)
hardly ever 20% (casi nunca)
seldom/almost never 10% (rara vez)
never 0% (nunca)
Frequency adverbs can go in the following positions in a sentence:
Before the main verb:
Subject + adverb + verb + object, etc.
He never eats vegetables.I often buy foreign periodicals.They frequently visit us.I always read commercial news.They rarely watch sports channels.I sometimes watch English films.
After the verb "to be":
Subject + be + adverb + complement, etc.
He is always late for office.We are never invited to dinner parties.I am always worried about my finances.He is sometimes consulted by investors.You are seldom anxious about my health.
Between the auxiliary and the principal verb:
He has always done justice to all.I have often thought of starting a business.I have never forgotten those unfortunate events.I have sometimes managed to hoodwink others.We could hardly ever appreciate his conduct.We shall never seek such favors.They will always regret having done this.They will usually take such big risks.
Some frequency adverbs (e.g. usually, normally, often, frequently, sometimes and occasionally) can also go in the beginning or end of a sentence.
Some other examples are: generally, seldom, rarely