Learn Hardcore Finnish: Lattia on kylmä aikaisin aamulla. - The floor is cold early in the morning.

Because lattia is the subject of the sentence. In Finnish, subjects normally take the nominative case, which for singular common nouns is simply the basic dictionary form (no extra suffix).

Why doesn’t kylmä carry a case or number ending here?

Here kylmä is a predicative adjective describing the subject lattia. Predicative adjectives agree with their subjects in case and number. Since lattia is singular and in the nominative, kylmä remains in its base form (nominative singular).

What form is aikaisin, and why doesn’t it look like an adjective?

aikaisin is an adverb meaning “early.” The adjective aikainen (“early”) takes an -in ending to turn into an adverb. Adverbs in Finnish don’t take case endings; they simply modify the verb or express time, manner, etc.

What case is aamulla, and why is it used to express time?

aamulla is the inessive singular of aamu (“morning”), formed by adding -lla (“in”) to the stem. Finnish uses locative cases like the inessive to indicate when something happens:

aamu (morning)
aamu + -lla → aamulla = “in the morning”

Could I start the sentence with Aikaisin aamulla instead of ending with it? Does that change the meaning?

Yes—you can say Aikaisin aamulla lattia on kylmä. Finnish has relatively free word order. Moving Aikaisin aamulla to the front shifts the emphasis onto the time (“Early in the morning…”), but the core meaning remains the same.

How do I ask “Is the floor cold early in the morning?” in Finnish?

Invert the subject and the verb:
Onko lattia kylmä aikaisin aamulla?

Onko = question form of on (“is”)
lattia stays nominative as the subject
the rest remains unchanged

How do I make the negative (“The floor is not cold early in the morning”)?

Use ei plus the negative verb stem:
Lattia ei ole kylmä aikaisin aamulla.

ei = negative marker for 3 sg.
ole = negative form of olla (“to be”)

How would I say “The floors are cold early in the morning” (plural)?

Make subject, verb and adjective plural:
Lattiat ovat kylmät aikaisin aamulla.

lattiat = nominative plural
ovat = 3 pl. of olla
kylmät = nominative plural adjective

Is there a difference between aikaisin aamulla and varhain aamulla?

They’re nearly synonymous. aikaisin aamulla (“at the earliest in the morning”) often suggests the very first hours, while varhain aamulla (“early in the morning”) is slightly more general. In everyday use, they can usually be swapped without changing the meaning.