Learn Finnish: Kotka on iskemäisillään. - The eagle is about to strike.

You take the strong verb stem iske-, add the fifth infinitive marker -mäisillä-, and attach the third-person possessive suffix -än. Notice that the final vowel of the suffix blends with the ending, making it -illään.

Why is the possessive suffix -än necessary at the end of iskemäisillään?

The fifth infinitive construction always requires a possessive suffix to reflect whose action it is. Because the subject kotka (the eagle) is a third-person singular noun, it takes the third-person suffix -än (which assimilates into the preceding vowel).

Could I just say Kotka aikoo iskeä to mean the same thing?

Not quite. While aikoa means to intend or plan to do something, the fifth infinitive iskemäisillään is much more immediate. It means the action is imminent, happening in the next split second, like being physically poised to strike right now.

Why does the ending use -mäisillä- with the vowel ä instead of -maisilla-?