Ich sah den ersten Rechner ?
Yes, that's fine. Report it if it wasn't accepted.
Thanks
I've reported this one too
can you use simple past with this sentence - "sah" instead of "habe gesehen"?
Yes, you can.
With a few exceptions (like "sein"), simple past is restricted to written German.
People always say this, but I've heard loads of Germans use the simple past! I think German speakers use it more than they think.
Correct. We use it all the time.
Nicht wahrscheinlich, Alan Turing hat den ersten Computer erfunden 1936!
Die Chinesen haben den Abakus bereits in der Steinzeit erfunden
Why ersten and not erste?
I don't know about you guys. But I HAVE NOT learned past tense yet.