Learn Italian: Se russi, non dormo sodo. - If you snore, I do not sleep soundly.

Sodo is an adjective being used as an adverb to modify the verb dormire (meaning to sleep "soundly" or "deeply"). When adjectives act as adverbs, they are invariable. This means sodo will always end in -o here, regardless of the speaker's gender or number.

What other verbs commonly use sodo as an adverb?

Besides dormire sodo, you will very frequently hear lavorare sodo (to work hard) and studiare sodo (to study hard). You might also encounter piovere sodo (to rain heavily).

Wait, doesn't russi mean "Russians"?

It can! However, in this context it is the tu (you) form of the present tense verb russare (to snore). You have to rely on context to tell the difference, as a sentence meaning "If Russians, I don't sleep soundly" wouldn't make sense.

Why are both verbs in the present tense in this Se sentence?

This is a Type 1 conditional expressing a simple fact or routine. When a condition is a general truth, like "Whenever you snore, I do not sleep," Italian uses the present indicative in both the Se clause and the result clause.