Can I substitute white wine vinegar for balsamic vinegar in a steak marinade?
Balsamic vinegar is quite sweet, fruity and tangy, and not easily compared to other types of vinegar.  The substitutes will work better if you're replacing a relatively little bit, especially in a recipe that uses other strong flavors... but the more important the balsamic flavor is to the end result, the more the substitution will show through.  It still might be pretty good, just expect it to be different than the original.

One substitution I found that's a little more specific is one tablespoon red wine vinegar and half a teaspoon sugar, for one tablespoon balsamic, another one is a tablespoon of cider vinegar and half a teaspoon of honey.  It could be substituted with sherry vinegar and a little honey (though this recipe was "to taste" instead of specific ratios).  Your choice might depend on which vinegar or sweetener you have on hand, which one you like more, or which goes better with the rest of the recipe.

I also saw a substitute with equal parts lemon juice and molasses (or brown rice syrup) with a dash of soy sauce.  This is a little more work, but because of that I expect it's a closer match to the flavor profile (or else it wouldn't be worth the extra effort).