Originally Posted by Steven Lake
Right, since there's already one past tense verb in the sentence, you'd need to continue using past tense throughout it. The tricky ones are those which only have one word in the entire sentence which is tense related, and even more so if it's an arbitrary word such as is or was.
Its smaller unit then just a whole sentence right? So I could say
I WAS a dog but now I AM a cat
The dog is happy and the cat was sad