If I modify a patented idea by someone else, do I need to pay license feeds?

Have a look at the patent application of your competitor and look at the claims. This is the part that specifies what exactly they are protecting. If your product or technology includes all the things they have in any of their claims then this means that your invention is using their technology and you need a license.

But have a look at this example:

If your competitor's claim says: My invention is composed of A, B, and X
If your invention is composed of A, B, and Y = you are different
If your invention is composed of A, B, X, and Y = you are using the competitor's invention and adding a component of yours and you will need a license.

I tried to make it as simple as possible but i have to say that you might need to consult a patent attorney or an IP consultant in your country just to have a look at the competitor's claims and tell you whether or not your invention falls within their scope of protection.

NOTE: I edited the first paragraph to replace
"If your product or technology includes any of the things they have in any of their claims" with
"If your product or technology includes all the things they have in any of their claims". Thanks Steven for the note. This is what i meant to say in my example