senā

1. The rule

Feminine ā-stems (like senā “army”, kanyā “girl”, mālā “garland”) are marked by the long final ā in the bare stem. Their singular forms differ from the masculine a-stem in three systematic ways: (i) a -y- glide appears before vowel-initial endings, (ii) the stem vowel stays long throughout the dual and plural, and (iii) the genitive and ablative singular share a single form — senāyāḥ (Whitney §363; MacDonell §77).

2. How to remember

Two signatures crack this paradigm:

1. i The -y- glide. In the singular, every case from the instrumental onward inserts a -y- between stem and ending: sen-ay-ā, sen-āy-ai, sen-āy-āḥ, sen-āy-ām. The stem never meets a vowel ending directly. This glide is the feminine signature.
2. ii The dual sene. A single form covers both nom. and acc. dual — and the voc. sg. too. It comes from senā + ī fusing into -e. If you see a feminine word ending in -e, it's this paradigm.
3. iii Gen. sg. = abl. sg. Both are senāyāḥ. One less form to memorise — context distinguishes them.

3. Exercise — senā

Fill in the full paradigm. Pay attention to where the -y- glide appears.