Btukfyl

With the following codes

documentclass[12pt,a4paper]{report}





begin{document}





section*{Work Experience}

  begin{tabular}{r|p{13cm}}

 textsc{Sebt.2011-Now}

 & Teaching Assistant, Department of Mathematics,

  University of California, Santa Barbara.\

  & Delivered courses: ......\

  multicolumn{2}{c}{} \

  textsc{Feb.2011-May 2011}

  & Substitute teacher at ....%\

 end{tabular}







 end{document} 

I get

Where is the problem?

The column p{13cm} is too wide and produces an Overfull hbox.

I suggest to use tabularx with an X column type.

Moreover, instead of writing textsc{...} in every cell of the first column, you can create a new colum type this way newcolumntype{R}{>{scshape}r} with array package.

documentclass[12pt,a4paper]{report}

usepackage{tabularx}

usepackage{array}

newcolumntype{R}{>{scshape}r}

begin{document}

section*{Work Experience}

begin{tabularx}{linewidth}{R|X}

 Sebt.2011-Now

 & Teaching Assistant, Department of Mathematics,

  University of California, Santa Barbara.\

  & Delivered courses: ......\

  multicolumn{2}{c}{} \

 Feb.2011-May 2011

  & Substitute teacher at ....%\

end{tabularx}

end{document}