Sometimes it is hard for my students to remember the whole list of 7 letters after which we never have the letter -ы but we use the letter -и instead.
Especially it is very important for putting nouns into the plural form (банк – банки), adjectives (высокий, хороший), or feminine nouns in the genitive case (у девушки, у собаки).
So the list is: к, г, ш, щ, ж, ч, х.
And if it is hard for you to remember it, I decided to think of a funny little sentence to help you to remember.
Карл Говорит Шарлотте, Что Желает Хорошие Щи. –
Carl says to Charlotte that he wishes to have a good Shchi (traditional Russian cabbage soup).