"A hammer is an object that one uses to hit with."
Translation:En hammer er en gjenstand som man bruker til å slå med.
'som' when it's a relative clause, 'at' when it's a dependent clause.
In a relative clause 'that' refers to something in the previous sentence.
"He is the man that saved me"
"Han er mannen som reddet meg."
In a dependent clause 'that' just starts the new sentence, which depends on information in the previous one, and 'that' can often be omitted.
"I know that it's green" = "I know it's green"
"Jeg vet at den er gønn"