There's a few possible endgames for Ukraine. The best scenario for Ukraine is that they're able to check the Russian invasion, and prevent troops from reaching

There's a few possible endgames for Ukraine. The best scenario for Ukraine is that they're able to check the Russian invasion, and prevent troops from reaching and occupying Kyiv and other major cities, causing the war to drag on in a stalemate. If it goes well for them, the Russian will to press the invasion collapses, and they withdraw without winning any concessions. Note that I do not think this is at all likely. The more likely scenario (that ends "well" for Ukraine) is that Russia successfully pushes its invasion to completion and installs a new government, but the invasion shifts to a phase of intense resistance (see, e.g., Syria, Iraq, Afghanistan) that ultimately collapses the moment Russian troops pull out, perhaps with similar speed as the Afghani government collapsed after the US's withdrawal. Both of those scenarios assume that a combination of economic sanctions and unpopularity of the war are sufficient to prevent Russia from maintaining troops in Ukraine indefinitely, which is an assumption that may or may not be true (and I think there are very few people who are qualified enough to credibly opine on its truth!). If the assumption is wrong, then the conflict might stabilize in a conflict that looks like Nargano-Karabakh (~30 years), Cyprus (~50 years), or Korea (~70 years). It's possible to theorize about a scenario in which Ukraine does so well that they're able to counterattack and retake control of Crimea and the Donbass, or even punch into Russia, but such a scenario is so unlikely that I think it should rightly be considered fantasy.