Translation:If I had a keyboard, I could use this computer.
I'm quite used to seeing the particle "ĉi" come before its corresponding correlative when reading/interacting with other Espernatists out in the wild, but this particular set of lessons has tended to be more liberal with its ordering. What are the rules governing when/where/why to break the norm on "ĉi+x"?
Someone please correct me if I'm wrong, but I don't know of any rule that governs whether the order ought to be tiu ĉi or ĉi tiu --- it seems to be just a matter of preference, not of differing meaning. Maybe the ĉi tiu order is simply "winning" these days. (I share your impression that I think I've seen more ĉi tiu than tiu ĉi)
-us isn't the past tense ending but the conditional. "If I had a typewriter" doesn't refer to something that happened in the past, but to something hypothetical, which would be nice but isn't actually real. This is the conditional. By chance it happens in English that the past tense and the conditional have the same ending (they may not always necessarily have been the same, but this is the way the language has developed over the centuries), so it's easy to get them mixed up. No risk of that in Esperanto - very definitely two different endings!