Keyword analysis takes words such as prepositions and articles. How to avoid that?

Hello,

I am optimizing my site for SEO and when I use tools that do semantical and keyword analysis, they detect keywords such as prepositions and articles. Obviously, I don't want that to occur.

Is that normal? If that is normal, do search engines, such as Google, take those kind of words off the keyword list?

If it is not normal, how can I avoid it?

Thanks Jaime