Grammar: Third Declension Nouns
The Latin third declension is the most frequently found of the noun declensions. It is essential that you be famliar with the endings of this declension.
In addition, you also need to make sure you know the stem of each third declension noun. You cannot necessarily guess the stem from the dictionary form of the verb (nominative singular). In order to identify the stem of the noun, you absolutely must learn the genitive singular form of the noun as part of your vocabulary.
What about i-stems? In Latin textbooks, you are often made to suffer over the distinctive between third declension i-stem nouns. This is a pretty silly thing to worry about. So, please do not worry about this! For the purpose of reading Latin, you just need to be prepared for an "i" that comes and goes in the endings of the third declension. No problem. And for Latin composition purposes, the world is not going to come to an end if you forget to include an "i" in an ending. (It matters not a jot or a tittle, so to speak.)