How to write 1, 2, … as a subscript to a letter?

Question

I have two theta angles, one is \theta -1, other one is \theta -2. How can I write that properly? I want to give numbers to the thetas, putting the numbers at the bottom right end of the letter.

Answer 1

This is called subscript and is activated (in math mode) with _:

\theta_1
\theta_2

You might (should) be interested in further reading, I recommend the Not So Short Introduction to LaTeX2e (surely available in your language).

MWE (some examples)

\documentclass{article}
\begin{document}\noindent
\verb|\theta_1|                    gives: \( \theta_1    \)                 \\
\verb|\theta_2|                    gives: \( \theta_2    \)                 \\
\verb|\theta_12|                   gives: \( \theta_12   \)                 \\
\verb|\theta_{12}|                 gives: \( \theta_{12} \)                 \\
\verb|\theta^1|                    gives: \( \theta^1    \)                 \\
\verb|\theta^1_2|                  gives: \( \theta^1_2  \)                 \\
\verb|\theta_1^2|                  gives: \( \theta_1^2  \)                 \\
\verb|\theta_{x,y}^{\frac{1}{2}}|  gives: \( \theta_{x,y}^{\frac{1}{2}}  \) \\
\end{document}

Output