why table row does not close?

Question

I am wondering why the row that ends {equations of motion} does not close like the one above it.

\documentclass{article}

\usepackage{graphicx}
\usepackage{multirow}

\begin{document}
    \begin{table}[htbp]
      \centering

    \begin{tabular}{|c|c|c|c|}
            \hline

            \multicolumn{4}{|c|}{{\bf Name and theme of lesson:} Equations of motion}\\\hline
            \multicolumn{2}{|c|}{{\bf Subject area:} Linear motion & {\bf Class }RTU & {\bf No.}1\\ 
            \multicolumn{4}{|c|}{\bf Lesson objectives} Derive equations of motion\\\hline
            \multicolumn{4}{|c|}{\bf Material and resources:}Whiteboard, Marker\\...

      \end{tabular}
    \end{table}
    
\end{document}

Answer 1

You should use

\multicolumn{4}{|c|}{{\bf bold text} normal text} \\

instead of

\multicolumn{4}{|c|}{\bf bold text} normal text \\

That is, the "normal text" should also be part of the 3rd argument of \multicolumn.

Full example, with {\bf ...} replaced with \textbf{...}

\documentclass{article}
\usepackage{multirow}

\begin{document}
\begin{tabular}{|c|c|c|c|}
  \hline
  \multicolumn{4}{|c|}
    {\textbf{Name and theme of lesson:} Equations of motion}
    \\ \hline
  \multicolumn{2}{|c|}
    {\textbf{Subject area:} Linear motion} 
    & \textbf{Class} RTU 
    & \textbf{No.}1
    \\ \hline
  \multicolumn{4}{|c|}
    {\textbf{Lesson objectives} Derive equations of motion}
    \\ \hline
  \multicolumn{4}{|c|}
    {\textbf{Material and resources:} Whiteboard, Marker} 
    \\ \hline
\end{tabular}
\end{document}

Answer 2

With tabullaray package, for exercise:

\usepackage{tabularray}

\begin{document}
\begin{tblr}{hlines, vlines,
             colspec={lcc}
             }
\SetCell[c=3]{l} \textbf{Name and theme of lesson:} Equations of motion 
                                        &   &                           \\
\textbf{Subject area:} Linear motion    &   \textbf{Class} RTU
                                            & \textbf{No.} 1            \\
\SetCell[c=3]{l} \textbf{Lesson objectives:} Derive equations of motion
                                        &   &                           \\
\SetCell[c=3]{l} \textbf{Material and resources:} Whiteboard, Marker
                                        &   &                           \\
    \end{tblr}
\end{document}

• as you can see, all lines in table are defined in table preamble
• your table use only three rows, therefore in above MWE are defined three columns instead of four in your MWE
• \SetCell[c=3]{l} ... is equivalent to multicolumn{3}{l}{ ... } in classic tables, however it consider vertical lines defined in table preamble