Czech grammar takes practice to master. If you need to refresh your knowledge of Czech grammar, try looking at David S. Danaher's "Outline of Czech Grammar." As the professor of Czech languages at the University of Wisconsin - Madison, his detailed outline is both reliable and useful for quick, grammar reference questions. The outline ranges from the basic principles of Czech grammar to the more advanced.
If practice makes perfect, then you will find the following resources helpful and enjoyable. Practice Czech grammar with this resource:
Slovnik.cz represents the most reliable, free online dictionary for the Czech language. In addition to English, slovnik.cz provides dictionary services for multiple other languages. It is compiled and maintained by Martin Vít. Detailed information about the website can be found through the links on the right side of the homepage.
The Czech Sounds Tutorial (Česká abeceda) provides pronunciation for individual letters and examples of each letter in common words. Additionally, there are sound tutorials on common vowel clusters, Czech names, adjectives, and common phrases used in the Czech classroom.
Having trouble pronouncing a word? Try Shtooka. This French website provides pronunciation for many Czech words, but does not provide dictionary services.
The Little Czech Primer is compiled and maintained by a librarian in Ohio. Although it is not comprehensive, it includes audio pronunciation, pictures, and verb conjugation. Additionally, the site maintains a news page with interesting updates and an online resources page.