One of the uses of the word чтобы is to indicate the purpose for which an object or action is designated. It is often translated into English as “to” or “so that”:

Notice that in those sentences чтобы was followed by an infinitive phrase. That's not always the case:

So what's the difference? The difference is who the “doer” is in the чтобы portion of the sentence. If the doer implied by the чтобы portion is the same as the doer in the main clause, then it is followed by an infinitive. If the doer is different, then you use чтобы to introduce a clause with a past tense verb. Note, however, that even though the form of the verb is past tense, the meaning is not necessarily past tense. It's just indicating the purpose of the action. Here are a couple more examples.

Today we will talk about another use of по, and this one is a weird one. When you express the idea of giving each individual in a group of people the same quantity of something, you express that quantity after the preposition по. In English we express the idea with the words each or apiece:

You will notice that in those sentences the dative case was used after по. You are probably thinking, “No problem. I'll just memorize that по in the distributive sense is used with the dative.” You poor sucker. The Russian language is about to kick you in the teeth. Consider the following sentences:

Pop quiz: what case is used after по when more than one object is distributed? Since два щенка and две кошки occur after по, any normal human being would say that it is the nominative case. There was an old assumption, however, that no preposition can govern the nominative case, which is mostly true in Indoeuropean languages. To be frank, the issue is complicated for reasons that are beyond the goals of this blog. The practical answer is this: when a number other than one is used after по, the number is in the nominative case. The noun that number quantifies shows up in the genitive singular if the number is полтора/ы, два/е, три or четыре, and in the genitive plural for other numbers. So here is a rule of thumb for first- and second-year Russian students:

Distributive по: uses the dative case when talking about a single item; uses nominative case of numbers other than one.

If you want to pursue the question of what the case actually is that по governs, start with Igor Melchuk's excellent book ... Melchuk is a grammatical god to whom I have erected a shrine (along with his publisher... ) in my home. I make a small offering to them every day in a brass bowl. The fire is fueled by pages ripped from People Magazine, which richly deserves to be burned, and the sacrifice is made of my own blood and hair, accompanied by me singing «Я хочу быть с тобой» by Nautilus Pompilius.

We previously discussed the use of the preposition по in the sense of “on the subject of.” Another common meaning of по is to indicate the means by which information is communicated, in which case it is usually translated as “on” or “via” in English; in that meaning по governs the dative case:

Don's rant of the day: the idea that we are supposed to capitalize the word Интернет is completely assinine. I refuse to do it in English, no matter what the current stylistic guides say. Nowadays the internet is a generic means of communication, not a capitalizable company like MCI. Lower-case makes sense. But if you are someone trying to get a job somewhere and not get rejected because of ignorami professing current political correctness, then capitalize the word.

But in Russian I'll capitalize it cuz I don't wanna seem like an ignant furriner.

The word но means but. In grammatical terms it is a conjunction which usually introduces a clause that is contrary to previously established expectiations. For instance:

The word но rarely produces confusion for English speakers. Bummer. That means I have to think of some clever example sentences.

Crud. I guess my sentences weren't so clever after all.

There's a quirk to spoken human languages: the shorter the word, the more meanings it has, and the longer a word the fewer meanings it has. For instance, “jack” has dozens of meanings, whereas “pneumonoultramicroscopicsilicovolcanoconiosis” really only has one (or perhaps none, if one assumes the word is never really used in anything but lists of long words). The same is true for Russian. The long preposition накануне has only two meanings, whereas the short preposition по has dozens. Today we will discuss по in the meaning of “on the subject of.” With that meaning по is used with the dative case. First of all, it can be used with nouns that indicate written content, e.g. books and magazines:

The preposition is also used for lots of things associated with academics, e.g. exams, textbooks, handbooks: