They are both in the instrumental case.
вилкой means that you are eating using a fork.
с вилкой means that you are sharing the table with a person wearing the fork costume:
I think I see the pattern:
с[о]+instrumental: "with" as in "accompanied by; joined by; the other parties in a conversation. Я говорю с девушкой.
"Instrumental case, no с: "with" as in "by means of; using; via." Я говорю языком.
Is that correct, more or less?
With "с", yes. But the instrumental case also works with some other prepositions.
Instrumental on it's own
According to a website about the Russian grammar, there are more than ten meanings (semantic roles) of that case:
- tool, instrument: есть ви́лкой / to eat with a fork, писа́ть черни́лами / to write with ink
- patient: владе́ть компа́нией / to own a company
- agent: дом, постро́енный мои́м де́душкой / the house built by my grandfather
- effector: кры́шу сорвало́ ве́тром / the roof was blown away by the wind
- cause: боле́ть гри́ппом / to be sick with flu
- path, trajectory: е́хать ле́сом / to go through the forest
- stimulus: любова́ться цвета́ми / to admire flowers, па́хнуть духа́ми / to smell of perfume
- time: ра́нним у́тром / early in the morning
- degree: вози́ть во́ду бо́чками / to carry water in barrels
- aspect: он краси́в душо́й / he has beautiful soul, страна́ бога́та ресу́рсами / the country is rich in resources
- comparison, orientation: крича́ть сово́й / to scream like an owl, стоя́ть столбо́м / to stand like a pole
- measure: у́лица длино́й два киломе́тра / a street two kilometers long
- to begin, continue, finish with, etc: день зако́нчился у́жином / the day was ended with dinner
- to be, become, turn out, etc: стать арти́стом / to become an actor
- many more
Instrumental + prepositions с, над, под, перед, за, между
- говори́ть с дру́гом / to talk with a friend
- стоя́ть ря́дом с до́мом / to stand near a house
- стоя́ть над землёй / to stand above the ground, работать над кни́гой / to work on a book (figurative)
- стоя́ть под мосто́м / to stand under a bridge
- стоя́ть пе́ред до́мом / to stand in front of a house
- стоя́ть за до́мом / to stand behind a house
- стоя́ть ме́жду до́мами / to stand between houses
Concretely "Я говорю языком" is bad Russian; if you want to say that you pronounce words using your tongue inside your mouth, "Я говорю с помощью своего языка́" or "Я говорю, испо́льзуя (adverbial participle) свой язык" are better, but such a phrase sounds strange anyway. Or you mean something else?
Nah, I was just trying to come up with a parallel use of "говорю", and "tongue" was the first thing I thought of when asking myself "what do you use to help you speak?"
Thank you for the detailed guide on instrumental cases, though! I'm definitely keeping this for later reference.
I could make some mistakes so tell me if you find one. Also, I added "рядом с", this is also instrumental.
To make your life easier, you can consider a noun in the instrumental case an instrument of an action in many cases:
to eat with a fork, to write with ink, built by grandfather, to blow away by wind, to carry water in barrels (nearly:-)
A better example: «Я говорю́ гро́мким го́лосом.» = “I speak with a loud voice.”
Can't comment on the rest, but иду implies movement on foot. Еду would be for transport.
I answered: "I eat with fork. " DL said the correct answer is: "I eat with null fork." I do not understand anything. Can anybody explain it.
"Null", in computer terms, is usually referring to a missing value. It's just a Duolingo glitch. You probably should report it.
You should use an article in English though - either with a fork or with the fork.
I dont know where we use such a sentences like this I have been learning Russian on Duolingo around a year, and I can't make a good conversation.
"Eating a fork" would be "я ем вилку", which sounds pretty different because of the "u" sound.
С means "with" but in the sense of "along with" or "together with". As if you and fork sat down and had a conversation over coffee.
If you are just identifying the tool you are using there is no preposition (for instance "I am writing in pencil" would be Я пишу карандашом)