Vowel Assimilation

Vowels may assimilate, depending on their length.
The following types can be distinguished:

α, α:, ο, ω, ε, e, ι, ι:, ι., υ, υ:, ει, οι, αι, υι, ου, αυ, ευ

Here the : marks longer vowels. The ι. is a special type, as it is short but strong; it is used e.g. in the verb suffices -ιc, -ι and -ιтε.

They can be categorised according to their length/strength:

ε
α, ι, υ
o
α:, ω, e, υ:, ει, οι, αι, υι, ου, αυ, ευ
ι:, ι.

The first two rows are weak, the last two are strong.

Assimilation occurs when a weak and a strong vowel meet.
(This is only a rule of thumb, though.)

E.g.:
αα - αα
α:ε - α:
α:ω - α:ω
αει - αι

The ο is medially strong, e.g.: αο - ω or eο - eο (both strong), but οω - ω (weak).
The ι: never assimilates (e.g.: ι:α - ι:α), and the ι. 'eats' all weak vowels, including the ο (e.g.: αι. - ι. or ωι. - ωι.).