"Men and women usually sleep in a bed."
Translation:Viri et feminae in lecto dormire solent.
"Solent" is not an adverb. "Solere" is the verb "to be accustomed to". And when there is a two-verb construction, you need the infinitive followed by the conjugated verb, the exact reverse of English. Now, in English we don't have a single verb that means "to be accustomed to" and we always follow that with a noun or a gerund, but it's the same basic syntax.
He wants to drink ~
They want to drink ~
We want to drink ~
The dogs are used to sleeping on the floor ~
Canes in pavimento dormire solent.
The dog is used to sleeping on the floor ~
Canis in pavimento dormire solet.
I am used to sleeping on the floor ~
In pavimento dormire soleo.
An infinitive doesn't have an ending that tells us anything about the subject. It can also sometimes itself be the subject.
Dormiunt is the third person plural "they sleep", while dormio is the first person plural "I sleep". Both tell us about the subject. in lecto solent dormio would be like saying "They usually I sleep in bed".
In lecto dormire soleo.
Think of "soleo" as working like "I want" in English. "I want to sleep in a bed" versus "He wants to sleep in a bed". The verb "to want" ("I want, you want, he wants etc) goes with the infinitive of the action "to sleep" regardless of who is wanting to sleep. Latin has a bunch of verbs which work like this, but English often uses adverbs like "usually" instead.
The thing about it is that "usually" isn't a great translation for soleō, because it is a verb and not an adverb. The sentence is more directly translated as "I use to sleep on a bed" or "I have the custom of sleeping on a bed." This way you see that you have to conjugate the soleō to agree with the subject, and not the dormiō.
So, like Moopish explains, as well, to express "I usually sleep on the bed" you'd say in lectō dormīre soleō.