Bridget, the short answer is yes, it's okay to mix tenses, even in the same sentence.
Tense helps to convey time in our stories. There are several different tenses, but the variations can be clumsily lumped into past tense (I walloped), present tense (I wallop), and future tense (I will wallop). (The variations include phrases like "I was walloping", "I might wallop", "I have walloped", and others.)
For those of us who refuse to break grammatical rules, changes in tense are perfectly acceptable in a given passage if different times are described in that passage:
I wallop for a living. I first walloped when I was sixteen, and I will wallop until I retire.
For those of us who happily break the rules, tense can be used to create certain dramatic effects, even if they aren't strictly correct. In Light In August, William Faulkner frequently changes tenses, with the idea--so critics say--that the sudden shift from past to present brings more immediacy to the story:
He drove on, the wagon beginning to fall into its slow and mileconsuming clatter. Neither does he look back. Apparently he is not looking ahead either, because he does not see the woman sitting in the ditch beside the road until the wagon almost reached the top of the hill.
What do you all think of that?
I may be wrong (someone correct me if I am), but I think older stories were usually written in past tense. Only recently, with writers like John Updike, did present tense prose come into vogue. I personally adopted the present tense in my novel Rooster because I thought the story felt more exciting that way, like it was unfolding in the moment. But, be warned, writing in present tense third person POV can bring out an unsettling side effect when you are doing public readings. Most of your verbs will end in S, and those S's sure like to call attention to themselves. Suffering succotash.
Questions: What tense do you prefer to write in and why? Have you used tense shifts for dramatic purposes? Did it work?