In a medium saucepan cook the potato and carrots, covered, in a small amount of boiling water until just tender, about 7 to 8 minutes; drain and set aside. If desired, heat taco shells according to package directions.
Meanwhile, in a large skillet heat oil; cook and stir onion and garlic in hot oil over medium-high heat for 2 minutes.
Add zucchini and corn; cook and stir 3 minutes more. Add chili powder, salt, and pepper. Cook and stir for 1 minute more. Add potato-carrot mixture and tofu. Stir gently to heat through.
Fill taco shells with vegetable mixture. Sprinkle with cheese. If desired, serve with salsa, green onion, sour cream, and avocados. Makes 6 servings.