Grammar note: The shoresh (root) of this word is נתן (to give as a gift).  The first nun is absent from this form because of assimilation — i.e. that, being a weak letter, it has transformed into a dagesh (doubling dot) in the following letter (taw) because of the addition of the mem prefix.  This is common in Hebrew.  Third-position heys tend to apocopate (disappear), and first-position nuns like to assimilate.