I am studying calculus from a book which has 8 chapters. The book starting with explaining the subjects, and during the book there are problems with answers that is about the subject. and also at the end of each chapter it has for example $15$ to $40$ exercises with answer next to each problem.
I studied chapter $1$ which is about complex numbers and I am doing the exercises at the end of the chapter. I am on question $14$ out of $35$ questions. I am doing well with exercises and trying to solve problems without looking the answers which comes next to the problem.
I am a little nervous about doing all the exercises after studying the subject because I thing maybe I forgot what I studied on complex numbers after I reach for example chapter $5$ of the book, because I do all the exercises after I studied it.
What is the best way to study such a book? Do you recommend to do all the exercises after studying the subject to dominate the subject or it is better to study the next chapter (which is sequences) and then go back to the problems of the complex analysis? I ask because I think it is important to know when self-studying math. Thanks.
If you are asking the question "what is the best way to study Mathematics?" then unfortunately I don't think anyone can give a truly useful general answer to this, as it will completely depend from person to person (which, without looking - is probably why you got a close vote. Opinionated questions aren't really for this Stack Exchange).
The only good piece of advice (that I can think of at least) is that you should avoid becoming a machine. What do I mean? Well - it's very easy to fall into the trap of just applying a set method over and over without ever really questioning what is going on deeper behind the scenes. I think this is also what leads a lot of people to believe Mathematics is "easy" (particularly at younger years), because students will get praised for such work. It's difficult, because students need to build an intuition for Mathematics in order to be able to think like a Mathematician - and by doing exercises over and over again like this it can eventually get you there, which is why it's a tricky thing. I'm not saying you should avoid doing problems where you just apply a method by any means - it's very important you do these. What I'm saying is that you should avoid applying methods without ever going deeper into it. Mathematics is a beautiful subject, and the most amazing exercises you will get are ones that force you to think for yourself, not do something mechanical like applying a set method over and over.
Now, you asked "will I forget insert area here by the time I get to chapter such and such?" - truth be told, it doesn't matter what level you are at in Mathematics, you will not remember everything. You may have a vague idea or rough feeling of how some proof you saw long ago goes, for example - but that doesn't mean you'll remember it crystal clear, and you shouldn't beat yourself up for this. You can't remember everything at once, and truth be told again I don't think you need to. You should take away the perspective from the problem, and let yourself ponder it. It will build your toolbox of problem solving skills. This sounds awfully "wibbly wobbly" and like a non-answer, but I do think we don't give our brains credit where credits true - subconsciously you remember a lot more than you think, and I believe this can help you solve problems without necessarily realising at times.