newcommand {mainv} {parnoindenthspace*{0pt}ignorespaces}

main {This text is written using command main}
mainv {This text is written using command mainv}

newcommand {main} {parnoindenthspace*{0pt}ignorespaces}

documentclass{article}

newcommandmain{parnoindenthspace*{0pt}ignorespaces}
newcommandmainV{vspace{-10pt}main}

begin{document}

main Some text
main Some text
main Some text
mainV some other, overlapping text
end{document}

% My standard header for TeX.SX answers:
documentclass[a4paper]{article} % To avoid confusion, let us explicitly
                                 % declare the paper format.

usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

makeatletter

newcommand*vspacebeforeline[1]{%
    ifvmode % if in vertical mode, act as "vspace{#1}"
        vskip #1
        vskip z@skip
    else
        @bsphack
        vadjust pre {%
            @restorepar
            vskip #1
            vskip z@skip
        }%
        @esphack
    fi
}

makeatother




begin{document}

Here is some nonsensical text, written for the sole purpose of filling up at
least one line.  Well, after all I~think that two lines would be even better.
And now, we use our new command exactly herevspacebeforeline{18pt plus 6pt
minus 6pt}, followed by a few other words to end the test.

vspacebeforeline{3pt}

It can be used in vertical mode, too.

end{document}