How do I say "I was going to" in German?
In example, "I was going to fall, but I grabbed on quickly." How do I say it in German?
yes, you can. Beinahe - almost - just makes it a bit clearer but isn't necessary to transfer the message
Sometimes you can translate the English to another expression. "I was going to fall" = "I was about to fall" = "I would have fallen" It turns out that the last one might be closer. ( "Ich würde fallen" is not right, but see the other comments here. I am surprised that you would use plusquamperfekt but it does make sense that it is in Konjunktiv II mood. It makes sense now that I think of it.) So per actual German speakers, you say “I had almost fallen....”
Sorry to burst your bubble, while you are right that some things can't be directly translated and that another sentence meaning the same thing may be easier to translate, the last one translates to "Ich währe gefallen". But still, good advise.
I agree with Saskia's answer, and I'd like to add that for "I was going to write him a letter, but he called me today, so I don't have to write to him anymore" you'd say, "Ich wollte ihm [eigentlich] einen Brief schreiben".