When can I leave out "der/dem/den" after the relative "wer/wem/wen"?

Question

Sometimes I see the *wer/was* relative clause followed by *der*, *dem*, and *den*. But sometimes it's not. For example (excerpted from [here](http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-wer-was.html)): > Wen ihr einladet, **den** müsst ihr auch empfangen. > Wen ihr einladet, müsst ihr auch empfangen. Looks like "den" can be taken out because we can identify the subject (ihr) and object (wen ihr einladet) and the sentence can be comprehensible without it. But I'm not sure. That's just my guess. The reason why I'm not sure is that every example in my textbook has *der/dem/den* after the w- clause (*wer*.../*was*...): > Wessen Herz rein ist, **den** habe ich lieb. > Wem du geholfen hast, **der** hilft dir auch. > Wen ich ehren möchte, **der** ist er nicht. > Wer den ganzen Tag arbeitet, **der** ist abends müde. If the distinction between the subject and the object is clear-cut, is it always allowed to leave out the demonstrative pronouns ("den" and "der" in the above cases) after the *wer/was* clause? Also, will the sentence below make the reader confused about who does what, without *der* or *dem*? (I mean, I wonder if this sentence can be interpreted in different ways without *der* or *dem*): > Wen ich gestern getroffen habe, schenkt ihr ein teures Bild.

Answer 1

My deceased German expert friend said decades ago that the der/dem/dens are fading out as their usefulness for clear communication is fading.

Hope this is helpful!