Hello, dear student! Today our topic is devoted to our favorite cases, which cause a lot of problems and mistakes to foreigners and Russian-speaking learners. The topic is quite difficult but we will try to split the information and provide you with the main situations when we use nouns in dative case with the preposition "по" [po] in our speech. Well,
1. To find out or tell about the profession or occupation of a person, we usually use the following phrases:
Кто вы по профессии? [Kto vy po prafesii?] - What is your profession?
Я по специальности врач [Ya pa prafesii vrach] - I am a doctor/I am a specialist in medicine
As these examples show, in situation when we talk about our profession or occupation, we use the preposition "по" followed by noun in dative case. The same concerns objects studied somewhere:
У меня сегодня шесть уроков по русскому языку [U minya sivodnya shest' urokaf po ruskamu yaziku] - I have six lessons on the Russian language course.
2. When we talk about ways of communication, i.e. we can talk with the help of telephone, skype, mail or the Internet. Here are also the ways of receiving the information - on TV and radio.
Таня услышала хорошие новости по радио [Tanya uslyshala kharoshie novasti pa radio] - Tanya has heard some good news on the radio.
Никита отправил письмо по почте [Nikita atpravil pis'mo pa pochte] - Nikita has sent the letter by email.
Родители услышали эти новости по телевизору [raditeli uslishali ekhti novasti pa televizaru] - The parents have heard the news on TV.
In all these examples the highlighted noun is in dative case which answers the questions "кому?"/"чему?" [kamu/chimu]. We always write the preposition "по" before a noun.
3. When we describe direction towards something, for example, a street, a roof, a tree, etc.:
Надежда шла медленно по улице [Nadezhda shla medlena pa ulitse] - Nadezhda was slowly walking along the street.
Кошки обычно карабкаются по деревьям [Koshki abichna karabkayutsya pa direvyam] - Cats usually climb the trees.
По крышам ходить всегда опасно! [Pa krysham khadit' fsigda apasna] - It is always dangerous to walk on roofs.
Thus, we have given consideration to the most common options and the situations when we should necessarily use the dative case. Of course, there are a lot of such situations, you should just start learning from the very beginning and deepen your knowledge.