I can't hear the difference between words ending in "и" or "ий". What is the difference?

This comment was originally written by E-chan. on Duolingo Russian forum in "I can't hear the difference between words ending in "и" or "ий". What is the difference?" thread.

I'm a native speaker and to me it seems that the difference in sound is indeed very subtle. The thing is that we just know what ending it should be. They mostly correspond to different word types. E.g. "-ий" would go with:
- adjectives (синий, общий, третий)
- participles (живущий, сказавший)
- many traditional male names (Василий, Дмитрий, Анатолий) and many surnames (some surnames end with "-и" but they are of foreign origin and not as common as surnames with "-ий")
- some pronouns (некий, всякий)
- some nouns (but you won't see them much, all examples I could remember are rather rare words like морфий, жребий)

"-и" would go with:
- adverbs (дружески, адски) and verbal adverbs (сказавши, увидевши)
- adjectives in plural instrumental case (синими цветами, старшими братьями, новыми окнами - the endings correspond with the nouns)
- verbs (иди, пиши, были)
- plural nouns (кони, дети), nouns in some cases (для кошки, днями) or some unchangeable nouns (такси)

This is just a general idea, strict grammar rules would be more complicated with additions and exceptions. But you asked for a tip and a tip is: for a start just remember for each word how to spell it. For the vast majority of words it can be only one of these endings possible. The big exception are adjectives vs. adverbs and participles vs. verbal adverbs.

A word can be an adjective and end on -"ий" or an adverb and end on "-и". Like дружеский/дружески, адский/адски. In this case you should think whether this word applies to the noun (then it would be an adjective) or to the verb (an adverb), e.g.:
дружеский визит (a friendly visit) - дружески позвать (to call smb. friendly)
адский шум (a terrible noise) - я адски устал (I am terribly tired)

The same difference is between participles (-ий) and verbal adverbs (-и). The former is a verb that is used as an adjective or a noun, the latter is a special form of a verb.
Сказавший это человек вышел - The person who said that has walked out. (The participle describes the noun like an adjective)
Сказавший это вышел - The one who said that has walked out. (The participle acts as a noun)
Сказавши это, он вышел - Having said that he walked out. (Here it works as a verb)