The missing letter rule assists the grammarian in finding the missing letter of the YQTL forms. (KHW 3.1, 6.1a, 7.1a, and 12.1) The key here is to take note of the vowel under the prefix:
Missing letter rule #1
If it is a sere, then the missing letter is from the beginning of the verb and is likely a yod (the verb הלך also fits in this category). This word is the verb ידע.
וַיֵּדַע
Missing letter rule #2
If it is a qamats, the missing letter will be from the middle of the root and will be a holem-vav, holem, or hiriq-yod (called hollow verbs). This is the verb שׂם.
וַיָּשֶׂם
Missing letter rule #3
If a root appears to begin with the same two letters (via a dagesh-forte), then the missing letter is an initial nun. The nun assimilated into the following letter; and hence, the dagesh-forte. Note in this verb how the word appears to have a tet for both its first and second letter because of the dagesh. This is the verb נטע.
וַיִּטַּע
Missing letter rule #4
If the vowel is anything other than a sere or qamats, then the missing letter is from the end of the root and is a ה. The word below is the verb עשה.
וַיַּעַשׂ