Hi! The instrumental case is used to describe the means by which an action is performed (I write with a pen). It is also used for the complement of a verb (I work as a doctor), in some time phrases, for nouns following some reflexive verbs and with certain prepositions. -The principal meaning of this case is ‘by/with’ to explain how an action is achieved. Ivan went to Moscow by train. He took photos with his camera. The preposition “with” must be used before the noun in the instrumental case: Я еду в Лондон поездом с сестрой. I’m going to London with my sister. -The instrumental case is used when a verb is followed by a ‘complement’ (which gives more information about the subject of a sentence). For example: Иван работает фотографом. Ivan works as a photographer. -The instrumental ending is found in time phrases relating to seasons and parts of the day: Летом in summer утром in/during the morning Зимой in winter вечером in/during the evening -Common reflexive verbs followed by the instrumental are заниматься (to be busy, occupy oneself) and интересоваться (to be interested in): Он интересуется рисованием. He is interested in drawing. To form the instrumental case: • Masculine nouns ending in a consonant add –ом, otherwise remove the las letter and add –ем: вечер (evening)→вечером, трамвай→трамваем, учитель→учителем. • Feminine nouns: remove last letter and add –ой to words which end in –a and –ей to words ending in –я or –ия. If a word ends in –ь don’t remove it, just add –ю. зима→зимой, Англия→ Англией, осень→осенью. • Neuter words: just add –м. утро→утром, здание→зданием.

In general, you should use it once you use 'with', 'by' or 'between' in English before the nouns. I was there with my friend. Я был там с другом. She was shocked with the news. Она была шокирована новостью. The сhef cuts meat with a knife. Повар режет мясо ножом. Eat your cake with a fork, please! Ешьте торт вилкой, пожалуйста! The law was rejected by the Parliament. Закон был отклонен Парламентом. The sculptor smashed his new sculpture by a hammer. Скульптор разбил свою новую скульптуру молотком. You make too big pauses between the words while speaking. Вы делаете слишком большие паузы между словами, когда говорите. Between sky and earth. Между небом и землей. War between countries. Война между странами.

I forgot the situation when a phrase like "Americans made it" or "Masha charmed him" is turned into passive, so verb changes into particuples: сделан американцами - made by Americans очарован Машей - charmed by Masha And I forgot many other situations:) But одним словом (in a word) it is adverbial case. It is 'how'.

4. It also used with preposition 'с': Мы с Машей - I and Masha, literally 'we with Masha'. This might have to do with its 'adverbial' role. It is not by chance that English uses 'with' to mean "means of action", 'with a hammer'. Я сделал это с помощью Маши. - I did that with Masha's help. Я сделал это с помощью молотка. - I did that with the help of a hammer. 5. It is used with 'за' in its 'locative' sense (behind/beyond). За дверью - behind the door. За холмом - beyond the hill. 'под' (under) под столом - under the table 'над' (above) над морем - above the sea

Valentin, if you had 72 more specific questions that would be easier:-) First it has "adverbial" meanings. Adverbs, e.g. "quickly" "yesterday" "here" in "I did that quickly" or "I did that yesterday", provide additional information about the action ("did"). 'How', 'when' etc. Instrumental does the same! 1. Means of action. It is the prototypical meaning of Instrumental, the first thing you think about when you think about the Instrumental: Забивать гвозди молотком. - with a hammer. In English that is expressed with "with" and "by". Заплатить рублями - to pay with/in roubles. Я добрался самолётом. - I got to there with a plane. Also: "Я добрался на самолёте". manner of action (or may be it is 'means' too?): Шел быстрым шагом. Сказал тихим голосом - said with a quiet voice. Допрыгнул одним прыжком. - with one leap. place of action: Ехал лесом. Was riding by the forest. This can be expressed with "ехал через лес" too. With some words like seasons or times of a day it can be time of the action: Утром - in the morning (but: "в прошлом году". With the word 'year' we rarely use it). 2 . Another 'adverbial' use is "as": Я помню тебя ребёнком. Я знал тебя ребенком. Я представлял тебя ребёнком. I remember / knew / imagined you as a child. or with another meaning of the same verb: Он представился Иваном - he introduced himself as "Ivan". 3 Because of its adverbial function it is very universal. We started to use it with some verbs where it doesn't answer the question "how": "казаться" - он кажется большим - it appears big. More literally: "it shows itself as big one". Or should "казаться" be in (2)? "становиться" - он стал космонавтом - he became an astronaut. "быть" - он был космонавтом, он будет комонавтом - he was/will be an astronaut. Nominative also can be used with 'become' and 'be', especially with adjectives!