"But would be okay if I said:
- I'm going to do a French course starting from tomorrow."
Yes, that's fine. It's just slightly more casual than your other choice.
What about textbooks? Let's suppose there's a textbook called RTK. Can I say:
- I took (studied and finished) this textbook in just 3 months.
No, you can't say that. It's incorrect.
- I took (studied and finished) the RTK in 5 months.
Again, it won't make sense if you say this.
- You should start doing (to study using) the RTK first before starting to learn grammar.
Yes, you can say this but it is informal. As well, you need to say 'the RTK textbook/book' and not 'the RTK'.
- I found it easier to learn vocabulary after taking the RTK.
No, this is incorrect.