How to change the definite name of figures? I tried
\documentclass[12pt,a4paper]{report}
\usepackage[czech]{babel}
\usepackage{amsmath, graphicx}
\renewcommand{\figurename}{\textbf{Obr.}}
\begin{document}
\begin{figure}[h]\centering
\includegraphics[width=\textwidth]{../img/struktura}
\caption{Struktura}
\label{s}
\end{figure}
\end{document}

\figurenameshould work (even without the float package) as always, your question would be clearer if it contained an example that demonstrated the problem. – David Carlisle Dec 20 '18 at 09:27\usepackage[czech]{babel}(in code you didn't show originally) which defines this for Czech so you need to use the babel declaration to set the Czech version of this command. – David Carlisle Dec 20 '18 at 09:35\addto\captionsczech{% \renewcommand{\figurename}{\textbf{Obr.}}and there is an error ! Paragraph ended before \addto was complete.}and the code you show in the comment has four{and only three}– David Carlisle Dec 20 '18 at 23:24captionpackage for example (and as noted in the linked answer here, you should not havetextbfin figurename` that should just be text, specify the formatting of the caption elsewhere) – David Carlisle Dec 20 '18 at 23:42x_{0}to get a subscript 0 – David Carlisle Dec 21 '18 at 13:55