Das, war vs es, war

Question

Which should I use and when:

das, was

or

es, was

I've seen both being used as 'what', e.g

Das, was ich habe, war schön.

What I saw was beautiful.

Which should you use?

Answer 1

Don't use "es, was" -- if you use "was", you don't need to put an additional "es" directly before it. In that case just use "was":

Was ich gesehen habe, war schön.
Schön war, was ich gesehen habe. (<== This sounds quite artificial though)

There are cases when you do need the "es", namely when "es" and "was" are not in direct vincinity. E.g. if you want to put the dependent clause after the main clause, but start the main clause with a subject:

Es war schön, was ich gesehen habe.

"Das, was" from your example is fine.

Wie hat dir Berlin gefallen? - Das, was ich gesehen habe, war schön.

There are some nuances here regarding what you want to stress. "Das, was ich gesehen habe" is the most explicit about the fact that you haven't seen it all and you're not sure if the rest is quite as beautiful, too.