Beth asked, "what is this weird slavic imperfective/perfective verb thing?"
But please don't blame her. And I did start to respond in the comments. I really did. But the whole thing is weird. (To me, that is. It's completely normal to the millions of native-slavic-language speakers who instead think the whole latin/germanic language system of tenses* is a bizarre perversion of the time-space continuum, or whatever. At least, that's what Mrs. Jana thinks. Tenses make her angry.) And because it's weird, it refuses to be confined to a brief comment.
So first of all, I may be making a gross generalisation. But friends who studied Russian used to complain about perfectives and imperfectives in that language. And now, here I am, learning a bit of Czech and lo and behold we hit chapter 12 (skinny chapters, btw) and they leap out to bite me in my linguistic hindquarters. So I googled, and it seems that verb aspects crop up in Bulgarian and Ukrainian in a big and important way too.
Anyhow, Sara-wearing-her-Rosina-hat could do this way better than me, but here's my amateur take on the matter after a couple of lessons:
Perfective and imperfective are two different aspects of verbs. "Aspect" kind of means how you look at/think about the action that's being described, regardless of when it takes place. So if you picture it from outside the action, in a linguistic out-of-body experience, it could be seen as a single, complete action. This is perfective (as in perfectly complete). If you picture it somehow from within the action (in other words, are more interested in the process as it's going on), it's imperfective.
Aspects are different from tenses, however, which is where it becomes confusing because I've definitely come across verbs in the past being called "perfect". So I've had to chuck that notion out of the window for the moment. In Czech, you can have past and present perfective (it has a future meaning), and past, present and future imperfectives.
So far, we're only dabbling in the past, which basically means deciding if you're interested in the fact that an action has been completed or in the action itself, which could sometimes give it a continous sense, but not always. It's not really like the imparfait/passé composé distinction beloved of French tests of yore which can usually be translated as "was/were yodelling" (imp.) or "used to yodel" (imp.)vs. "he yodelled" (pc). It also communicates things in ways that are expressed in English completely differently.
For example, you could say "Last Saturday that pudgy and over-familiar chimpanzee [shopped for] a new blouson leather jacket." If you used the perfective [shopped for] you could mean either:
1. He'd found and bought the jacket or...
2. He'd checked out all the jackets and decided that he'd rather get something in padded gingham and chains and spend the change on a cheap fedora.
But if you used the imperfective aspect of [shopped for], you could:
1. Be setting the backdrop for another action in the sense of "was shopping for…when he was attacked by a vengeful Confucius impersonator", or…
2. Mean that he'd shopped, not bought anything, and would probably go back and look at the discounts in the sales next week. Or not. You just don't care if the chimpanzee is stuck forever in limbo, still shopping. Given his taste in outfits that may be a good thing.
This is about as far as I've come. And the way Mrs. Jana nervously changes the subject whenever I mention the present tense gives me the feeling we're just skimming the surface. Basically it's just weird and depends on point of view and probably just getting a feeling for it.
But here's where it gets more weird. Because instead of just taking your average verb and twisting it in devious but ultimately classifiable ways (stick a bit on the end/the beginning/in the middle, oh but here's an exception) those wacky Czechs decided instead to use a COMPLETELY DIFFERENT WORD. Admittedly it's one that often appears distantly related, but only in the sense that some of the letters or the odd syllable are the same.
There are no rules about how it works on this frontier, kids. And I bet that as a beginner I'm getting the easy ones. Some verbs get longer, some get shorter. Some add a prefix or suffix, some change letters or syllables or any combination of these things. So you just have to learn them. Except there can be more than one perfective verb for an imperfective one. Not always pairs of verbs tromping along two-by-two in a metaphorical crocodile for the Czechs. Oh no.
Oh. And all verbs of motion, such as "going-by-feet" and "going-by-transport-including-roller-skates" are exceptions and are always imperfective, but it's okay because they form the future tense in a TOTALLY DIFFERENT WAY.
All of which makes me suspect that those Czech grammarians had a lot of repressed anger.
Sorry Beth. You give me garlic-browning tips, I give you rants on linguistics. Life is very cruel.
Edited to add: Aaaarrrgggghhhh. Verbs of motion are all perfective. Not imperfective. Aaaaarrrrgggghhhh.
*By this I mean things like, "I will wankle-rotate", "I will be wankle-rotating", "I will have wankle-rotated", "I'm going to wankle-rotate", the nuances of which I am woefully ill-equipped to explain. I just wankle-rotate away. Or is it off?