Weak Nominal Declension

The weak declension of nouns is nothing more than n-stem nominal formation. In principle it is no different from the formation of r-stems or nd-stems: the affix -Vn- intercedes between nominal root and endings, where V is some vowel. Just as brōþar 'brother' is inherently an r-stem noun (employing no other formations), likewise weak nouns are inherently weak, i.e. only n-stem. While adjectives may employ strong or weak declension according to contextual demands, a given noun by contrast employs only one declension at all times: a weak noun always declines weak, a strong noun always strong.

http://www.utexas.edu/cola/centers/lrc/eieol/gotol-4-X.html#Got04_GP16

an-jan-wan-stems

These nouns are typically masculine and neuter.

The zero-grade formation of certain nouns generalized the n-stem. The Vn-affix is essentially zeroed out, leaving the 'n' of the affix plus the appropriate case suffix.

It only affects some an-stems.

ón-jón-wón-stems

All of these nouns are feminine.

in-stems

All of these nouns are feminine.