Hebrew Verbs: Niphal
Nifal has specific prefix Ni. In the imperfect this prefix is assimilated with imperfect prefix in a way that the first letter of the root is doubled (dagesh).
Hebrew Verbs: Piel
Piel has doubled middle letter in its root (dagesh), in all its forms. In the imperfect shewa is present under the prefix.
Hebrew Verbs: Pual
Pual has doubled middle letter in its root (dagesh), in all its forms. In the imperfect shewa is present under the prefix. Its same as Piel except for regularity of u-a vowels