Can I use regular flour to bake in a Marble Loaf cake?!

yes but use a sieve for the flour to make it lighter and finer.

Regular All urpose flour has more gkutten than cake flour which effect the final crumb of the cake making it tougher. Cake flour make a tender crumb for the cake. You can try the sub I use.

Cake flour sub


cups
1 3/4 cups all-purpose flour
1/4 cup cornstarch
Change Measurements: US | Metric

Directions:
Prep Time: 2 mins

Total Time: 2 mins
1 To make two cups of cake-and-pastry flour (cake flour), combine 1 3/4 cups all-purpose flour with 1/4 cup cornstarch; proceed with your recipe.
2 The easiest way to do this substitution is to put 2 tbsp of cornstarch in the bottom of a 1-cup measuring cup, then fill the cup as usual with all-purpose flour and level top. Repeat process to get your two cups.
3 If you only need 1/2 cup of cake flour, put 1 tbsp cornstarch in bottom of 1/2 cup measuring cup, then fill and level as above

The answer to the question is yes, but the substitution is not one-to-one. The general rule of substitution is 1 cup of all purpose flour minus 2 tablespoons (1c – 2tbsp) is equivalent to 1 cup of cake flour. ....

Now, that said,I would highly recommend that you stick with whatever type of flour your recipe calls for, even when it means going out to the store to buy a box. The substitution is fairly reliable, but this is definitely a “substitute at your own risk” sort of thing. Different flours have different protein (gluten) contents and different weights, and different flours can result in dramatically different results in different recipes...

Good luck.........

To make cake flour add 2 tablespoons cornstarch for every 3/4 cup all-purpose flour and whisk to combine

the loaf will not rise as much as if you use cake flour.... not that big of a deal cooking is all about using what you got and finding out the results the hard way