> ...
> How do i insert a white space between (these) two values?

use <xsl:text> </xsl:text>
> 
> 2. Prob: Line Feed & Carriage Return
> The xslt:
> ...
> <person>
>    <xsl:if test="exists(mds:node17)">
>       <name>
>          <xsl:value-of select="node17"/>
>          <xsl:if test="exists(node18)">
>             (<xsl:value-of select="node18"/>)
>          </xsl:if>
>       </name>
>    </xsl:if>
> </person>
> ...
> This is the expected output:
> ...
> Lastname (Function)
> ...
> But this is the real output:
> ...
> Lastname
>               (Function)
> ...
> How can i control if a linefeed is added or not? Is there a 
> possibilty to remove or add a linefeed with xslt to the output?

Use <xsl:text>(</xsl:text>
> 
> I hope you have some advice for me.

A simple and safe rule is: don't have any non-whitespace text in your
stylesheet except within <xsl:text> elements. Use <xsl:text> whenever you
want to output literal text to the result tree.

Michael Kay
http://www.saxonica.com/