Just watched a Russian show today (The Road To Calvary), and I noticed they frequently add -то behind a word.
For example:
«Когда ж это будет-то»
«А потом-то вернетесь»
What does adding -то mean?
If I were to remove the -то from the word, is the sentence still correct? Would it mean a different thing?
This is a particle which puts some shared context or knowledge of the conversation as a theme (topic) of the conversation and expects some development (rheme) on that topic.
It's a description in very broad strokes, and the details differ between affirmative sentences and questions, so bear with me.
Here's a couple of examples for affirmative sentences:
This is a piece of watercooler gossip which can begin and end just like that.
Here, the second party is telling two things:
Sasha knowing that the fact that his wife is cheating is somehow already in context. Maybe it's common knowledge. Maybe it's not relevant to the discussion. Maybe the second party thinks that "Sasha knows" or "Sasha doesn't know" is an expected response (so it's it context for them).
The discussion should further elaborate how to deal with this knowledge. This further development is kinda expected, so the second party cannot just stop conversation like that, that would be awkward.
So in a sense, the second party says "OK, I know you're expecting to hear whether or not Sasha knows his wife is cheating. I'll tell you that he does, because that's what you're expecting to hear, but at the same time I want to underline that I assume this should be past discussion and develop on that"
Note that the topic of the conversation is not whether Sasha knows or not. The topic is "We have established that Sasha knows". It's a very fine line but it's still there.
For questions, the shared context may be something that the parties have been discussing previously. So using -то might bring that thing back as a topic and expect some development on it.
— Мы едем в Америку. — А потом вы вернётесь? // "We're going to America". "Will you come back?"
The question is just a request for information.
— Мы едем в Америку. — А потом-то вы вернётесь? // "We're going to America". "About what happens next: you're coming back, aren't you?"
Somehow потом is a shared context.
Maybe the parties had some plans together. Maybe they had been discussing the come-back plans a minute ago but the second party forgot the details.
The second party perceives what happens after the trip to America as a shared context of the conversation, and wants to bring it back to discussion.
They are saying: "Somehow, I assume there is some implied "after the trip" that you will understand once I bring that up. So I'm bringing it up. What happens "after the trip"? Are you coming back?"
Я-то понимаю. Ночь-то какая тёплая! Слушать-то слушал, да ничего не понял. Где-то он сейчас? Что дальше-то делать будешь?
It doesn't change the meaning though.
Кто-то звонит. Что-то упало со стола. Какой-то человек вошёл в дом. Когда-то слышала об этом. Где-то потерял очки. Куда-то убежали. Почему-то плачет.
Рассказывает: был там-то и там-то, делал то-то и то-то, встречался с теми-то.
A couple of examples:
In some questions with "when" it is used to show impatience. - "Когда они приедут-то?"
In some questions with "what" it is used to emphasize urgency. - "Что ж ты делаешь-то???"
In exclamatory aentences like "Красота-то!" it is used to draw special attention of the listener to something , for exampke, a sight.
In "А потом-то вернетесь?" it is used to show that the person asking is expecting them or hoping for them to return and asks to confirm that. When you get asked this question, you know that the person asking is expecting you to return, while in simple "A потом вернетесь?" you cannot draw assumptions whether they expect you to return or not.
"Что дальше-то делать будешь?" = "So what are you going to do next?", while "Что дальше делать будешь?" = "What are you going to do next?"
As native russian I can tell you that:
Answers by @tom-au and @quassnoi don't suit this particular case. So section 2-3 by @v-v also do not. Answer by @nick-the-dick is close but not exactly correct.
Section 1 by @v-v is correct but with some clarification: this particle indeed emphasizes (makes accent on applied word) to concentrate main target of a whole expression. Particularly when applied to a predicate it accents an action of an expression itself rather then other attributes.
Besides that if applied to non-predicates (some attribute words of expression) then this particular attribute becomes the main target of expression. That's why it's illegal to use more than one such construct in a single phrase. E.g. regarding questions particle "-то" makes clear what exactly is asked and important in the question.
The addition -то is used for emphasis. It does not change the fundamental meaning of the word. Even so, there are some subtleties involved.
For instance, the term кто-нибудь " means "someone," but "in particular." But кто-то means some "random" person.
