Simple sentence x Compound sentence

Hello guys! I was doing an online exercise about simple and compound sentences. The website told me two sentences I corrected were wrong.

"The basketball tickets are expensive, and we don't have much money to spend." (I put a comma before the conjunction)

"We ran to the end of the street and crossed to the other side." (I didn't put a comma before the conjunction because it's a simple sentence with compound verb so no need to do so)

Can someone explain me why? Isn't the first sentence a compound sentence and thus I have to use a comma? And the second sentence shouldn't have a comma because it's a compound verb?
MENSAGEM PATROCINADA Você sabe como está o seu nível de inglês? Teste agora GRÁTIS em apenas alguns minutos.

Clique aqui para iniciar o Teste Online!
Avatar do usuário gian2hard 2270 1 7 50
This might just answer your question!

1) Commas with compound predicates

A comma is not normally used between the parts of a compound predicate—that is, two or more verbs having the same subject, as distinct from two independent clauses. A comma may occasionally be needed, however, to prevent a misreading (as in the last example C).

A) e.g., He printed out a week’s worth of crossword puzzles and arranged them on his clipboard.
B) e.g., Kelleher tried to contact the mayor but was informed that she had stopped accepting unsolicited calls.


C) e.g., She recognized the man who entered the room, and gasped.

2) Commas with independent clauses joined by conjunctions

When independent clauses are joined by and, but, or, so, yet, or any other conjunction, a comma usually precedes the conjunction. If the clauses are very short and closely connected, the comma may be omitted unless the clauses are part of a series. These recommendations apply equally to imperative sentences, in which the subject (you) is omitted but understood (as in the fifth and last examples).

A) We activated the alarm, but the intruder was already inside.
B) All watches display the time, and some of them do so accurately.
C) Do we want to foster creativity, or are we interested only in our intellectual property?
D) The bus never came, so we took a taxi.
E) Wait for me at the bottom of the hill on Buffalo Street, or walk up to Eddy Street and meet me next to the yield sign.
F) Donald cooked, Sally trimmed the tree, and Maddie and Cammie offered hors d’oeuvres.


G) Electra played the guitar and Tambora sang.
H) Raise your right hand and repeat after me.

Reference: The Chicago manual of style, 16th edition