Math & Science ⇒ No counter 'Својство1' defined.

[Srcee]

[…] 1. Your answer solved my problem, but I have a new one. In my book, I have theorems and lemmas. But their numbering continues. […]

You can make such environments share one counter. See code below for the full example.

[…] 2. Which errors do you get? Are you compiling in Xelatex? Cause I do.
If you uncomment the line %\usepackage{amsthm} line, do you get any errors? […]

I compiled with XeLaTeX and got the same error. I must have mixed up something.

[…] 3. I tried with \setcounter but I don't know how it works. Can you give me a one-line example? […]

Just set the »chapter« counter before introducing the next chapter. See code below.

[…]4. Tnx for your answer but is there a way for doing this without installing new packages?

The package is not only for this purpose but also provides correct hyphenation for your document. So it is the preferable method to use it. Are there any reasonable arguments against the installation?

For 1 and 3: my problem is solved, thank you very much, I'm very gratefull.
For problem 4: I don't have time now, but I will try to install it one of these days and I'll write you again.

Again, thank you very much.