Why is the Czech orthography so important for a foreigners?
1/ The written form of the word can help you with the right pronouncing.
2/ Some words are different only in the written Czech.
3/ You should know every letter in a word to create the right forms.
Vyjmenovaná slova
The problem with a "Y/I" in the middle of a word. Even a Czech native speakers have to memorize some groups of the words - here is its list and exercises for a practice.
grammar (3 pages)
exercises (2 pages)