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.
This seems like it would be difficult to hear the ending of вилкой during natural, quick speech. Would most people just guess through context that the person is not actually eating a fork, but eating using a fork if the ending of the word was not very audible?
"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)