Well, this is probably a weak example, but this is what I mean by a sentence with single tense.
The cat was dead.
The cat is dead.
Multiple tense would be something like this.
The cat was dead because it was not alive.
The frog is not a reptile, but rather is an amphibian because of its nature.
Mixed tense therefore would be like this.
The door is open because it was designed to open automatically and will do so again. (past, present, and future in this example)
I hope that explains what I was saying. ^_^