The Eight Noun Rules (BBG page 346)
Rule #1:
Stems ending in alpha or eta are in the first declension, stems ending in omicron are in the second, and consonantal stems are in the third.
Rule #2:
Every neuter word has the same form in the nominative and accusative.
Rule #3:
Almost all neuter words end in alpha in the nominative and accusative plural.
- In the second declension, the alpha is the changed stem vowel; in the third it is the case ending.
Rule #4:
In the dative singular, the iota subscripts if possible.
- Because an iota can subscript only under a long vowel, it subscripts only in the first and second declensions.
Rule #5:
Vowels often change their length (“ablaut”). “Contraction” occurs when two vowels meet and form a different vowel or dipthong.
- λογο + ι = λόγῳ.
- λογο + ο = λόγου.
- γραφη + ων = γραφῶν
“Compensatory lengthening” occurs when a vowel is lengthened to compensate for the loss of another letter.
- λογο + .
- λόγος
- λόγους
Rule #6:
In the genitive and dative, the masculine and neuter will always be identical.
Rule #7:
The Square of Stops
- Labials + sigma form ψ; velars plus sigma form ξ; dentals plus sigma form σ.
- The ντ combination drops out when followed by sigma (παντ + ς = πᾶς).
- Whatever happens in the nominative singular third declension also happens in the dative plural. σαρκ + σ = σάρξ; σαρκ + σι = σαρξί.
Rule #8:
A tau cannot stand at the end of a word and will drop off. When no case ending is used in stems ending in -ματ, the tau drops out.
ὀνοματ – stem
ὀνοματ add the case ending (no ending is used in nominative singular neuter)
ὄνομα if no case ending, then the tau drops off
William D. Mounce, Basics of Biblical Greek: Grammar, ed. Verlyn D. Verbrugge, Third Edition. (Grand Rapids, MI: Zondervan, 2009), 346.