Learn German: Ein guter Staubsauger hilft sehr. - A good vacuum cleaner helps a lot.

This sentence is in the nominative case because the vacuum cleaner is the subject. The noun Staubsauger is masculine. Because the word for "a" (ein) doesn't have a special ending to show the masculine gender, the adjective has to step up and do the job. So, gut adds the masculine ending -er to become guter.

How can I tell that Staubsauger is a masculine noun?

Staubsauger is a compound noun ending in -er. In German, nouns that end in -er and describe a person or a device doing a job (like a cleaner or a driver) are almost always masculine, so they take der.

Why is the verb spelled hilft instead of helft?

The verb helfen (to help) is a strong verb that changes its stem vowel from e to i for the "he/she/it" forms. Since the vacuum cleaner is a third-person singular subject, the verb becomes hilft.

Doesn't sehr usually mean "very"? Why does it mean "a lot" here?

Yes, sehr translates to "very" when it comes right before an adjective (like sehr gut). But when it is used on its own to describe an action verb, it translates to "a lot" or "very much".