"Comma splice"

L'anglais n'emploie pas de virgule pour séparer des phrases grammaticalement séparées (= avec un autre sujet et/ou verbe). La faute est connue sous le nom de "comma splice".
Dans ce cas, il faut (selon le sens) employer un point, un point virgule ou deux points:

This computer doesn't make sense to me, it came without a manual. 
=> This computer doesn't make sense to me. It came without a manual.
Je ne comprends rien à cet ordinateur. Il a été livré sans mode d'emploi.

Hemingway was a great writer, he had a way with words.
=> Hemingway was a great writer.  He had a way with words.
Hemingway était un grand écrivain.  Il savait comment se servir des mots.

Mr. Brook has sent his four children to college, however, he has had to work day and night to earn the money for it.
=> Mr. Brook has sent his four children to college; however, he has had to work day and night to earn the money for it.
Monsieur Brook a envoyé ses quatre enfants à l'université; or, il a dû travailler nuit et jour pour gagner l'argent nécessaire.

The temperature became milder, spring had started.
=> The temperature became milder: spring had started.
La température devint plus douce: le printemps avait débuté.

This chapter has a lot of difficult information in it, you should start studying right away.
=> This chapter has a lot of difficult information in it: you should start studying it right away.
Ce chapitre contient des tas d'informations difficiles: tu devrais commencer à l'étudier tout de suite.