Simple c# for-loop program for calculating theoretical earnings from investing (beginner)
Here are some points that I think can make your code look good -


You can improve the readbility like you can group every set of operations with one line space. Like 

The local variables can be renamed better as they're not clear to me when I look at the code directly without reading the problem statement. For example inpMonth and inpMonthTrue are confusing.
You can include code comments on the logic that you're performing. For example in the below snippet, it is unclear what exactly is happening.  





The main() method can be broken into multiple meaningful methods to improve readability. Assuming that you'll be taking the input from the console



This was just a gist of what I'm trying to convey. It needn't be the exact names, you can create any number of methods for grouping the related logic and code. Breaking a big tall method into multiple meaningful chunks gives more clarity.
Plus, it is always helpful to detect any bugs that arise in the software, so you know where exactly to look into if the Interest is coming wrong for instance.