Abnormal 3rd declension patterns (Review Occasionally)

Question 1

paradigm for πιστις

Answer 1

πιστις
πιστεως
πιστει
πιστιν

πιστεις
πιστεων
πιστεσιν(ν)
πιστεις

Question 2

Paradigm for πατηρ

Answer 2

πατηρ
πατρος
πατρι
πατερα

πατερες
πατερων
πατρασιν(ν)
πατερας

Question 3

paradigm for ὑδωρ

Answer 3

ὑδωρ
ὑδατος
ὑδατι
ὑδωρ

ὑδατα
ὑδατων
ὑδασι(ν)
ὑδατα

Question 4

What is usually the vocative in the 3rd declension singular

Answer 4

1) usually the stem,
2) but sometimes the stem vowel will ablaut
3) the vocative of πατηρ is πατερ