Here are the correct forms for the verbs in brackets:

3. **flexibility** (noun form)
4. **assimilation** (noun form)
5. **borrowed** (past participle)
6. **creation** (noun form)
7. **effective** (adjective form)
8. **national** (adjective form)

So the completed sentence would read:

"Many people say, it's the **flexibility** of function of individual words, the **assimilation** of **borrowed** and the spontaneous **creation** of new words that have made English an **effective** medium of **national** communication."