The masdar get used when you talks about the origin of the thing,
when we use the verb, we can use the masdar instead. for ex.
-Mohammed trains alot (محمد يتمرن كثيرا) . or -Mohammed is doing alot of trainings (محمد يقوم بالكثير من التمارين) .
We can't say "Mohammed trainings alot" because it must be a verb not masdar.