A regular checking account simply lets you do all the things you'd expect from a checking account.

Although jellyfish stings occasionally kill humans, some jellyfish can actually benefit people.
Independent clause: some jellyfish can actually benefit people.
Dependent clause: Although jellyfish stings occasionally kill humans

Scientists have used drugs made from jellyfish to treat cancer and heart disease.
Complete verb: Have used
Helping verb: Have

Lifting bags of groceries. - is an example of muscular strength.

The movement of another swimming creature causes the stinging cells in the tentacles of a jellyfish to fire. -prepositional phrase

Medical research may find more uses for jellyfish in the future. - Linking verb

That night, representatives from major record labels and music television stations were in the audience at CBGB.
"Representatives" is a plural subject.

The group recently performed at CBGB, a club in New York City.
"In New York City" is a prepositional phrase.

Subject-verb agreement errors affect credibility in formal writing but may be overlooked in casual speech. Other mistakes, like misplaced modifiers or run-ons, can be more confusing. While important, they are not always the worst grammatical errors.