Can someone, somehow please explain what the differences are between the suffixes -то and -нибудь? They tend to decorate words like как, кто, где, что, когда́ as in
как-то, как-нибудь (somehow)
кто-то, кто́-нибудь (someone, somebody)
где-то, где-нибудь (somewhere, someplace)
куда́-то, куда́-нибудь (somewhere)
что-то, что-нибудь (something)
когда́-то (sometime in the past?)
когда́-нибудь (some time in the future?)
I'm not a linguist or a teacher, but this is how I understand it:
как-то is somehow, как-нибудь is also somehow, but it also can mean anyhow, in any way.
So, in the similar manner:
кто-то = someone, кто́-нибудь = anyone or whoever
где-то = somewhere, где-нибудь = somewhere, but not important where, someplace or wherever
куда́-то = somewhere, куда́-нибудь = anywhere
что-то = something, что-нибудь = anything
когда́-то = sometime, когда́-нибудь = some not specific (anytime) time
That's a really complicated question if you want every single of these discussed with examples
Long story short, -нибудь refers to a hypothetical object: you have an idea that something might fit your description, but maybe such object does not exist at all. If it exists, it does not quite matter, which one. Кто-нибудь and что-нибудь may also refer to a choice from a set of known objects when it does not matter which one is chosen.
-то refers to a real but unidentified object. It is a certain object, but you do not know what it is.
This makes them not interchangeable in some contexts.
In questions, assumptions and suggestions about the past it is OK to use кто-нибудь and что-нибудь. In statements, you cannot use them in the past. After all, if you are sure the event happened, then its participants are real—even if you do not know them:
In the future "кто-нибудь" is used more freely that "кто-то", because most "something/someone" sentences about future are suggestions, anyway:
"Когда" is time-related by itself, so its suffixed versions are quite different in usage.
когда-то is typically used in the past to refer to some distant times in the past, "once".
когда-нибудь is either used for the past ("ever") or for indefinite future ("some day"). If you are absolutely sure an event is bounf to happen, you can use "когда-то", too:
There exist a few additional meanings for a few of them:
как-нибудь may be used colloquially to refer to "someday, when you have time" etc.—for example, when inviting someone to hang out with you.
где-то is often used colloquially as "about, approximately", even though such usage is not acceptable for formal writing.
The way I used to describe the difference to an English speak is to draw a parallel with prefixes some- and any-, specifically:
кто-то / кто́-нибудь
somebody / anybodyгде-то / где-нибудь
somewhere / anywhereчто-то / что-нибудь
something / anything
It's slightly different with как-то / как-нибудь, and когда-нибдь / когда-то but the meaning stays the same:
Он как-то это сделал
He did it somehowКак-нибудь сделаем
We'll do it somehow
You correctly identified that -то would normally be used with passed actions and -нибудь with the future ones. In English this would also alter the meaning of the same -how word.
Russian "-нибудь" is like English "-ever".
кто-нибудь - whoever, где-нибудь - wherever, когда-нибудь - whenever etc.
The word "кто-нибудь" is derived from "кто ни будь" i.e. "кто (бы) ни был" - which means "whoever (was)". The others have similar origin and meaning.
