Talvella mustat sukkahousut ovat mukavat mekon alla.

In Finnish, case endings are also used for time expressions, not just places.

Here, talvella is the adessive form of talvi (winter), and with seasons it commonly means in/during that season:

• talvella = in winter
• keväällä = in spring
• kesällä = in summer
• syksyllä = in autumn

So this is a normal Finnish way to say in winter, even though the same ending -lla/-llä often has location meanings in other contexts.

Because the subject is plural: sukkahousut = tights / pantyhose.

In Finnish, words that describe the subject usually agree with it in number:

• mustat sukkahousut = black tights
• sukkahousut ovat mukavat = the tights are comfortable

So:

• mustat is plural because sukkahousut is plural
• ovat is the plural form of to be
• mukavat is plural because it describes the plural subject

Usually, yes in everyday use.

The dictionary base form is sukkahousu, but when talking about the garment you wear, Finnish very often uses the plural sukkahousut, much like English uses tights.

So a learner should normally treat it as a plural word in sentences like this:

• Sukkahousut ovat mustat.
• Nämä sukkahousut ovat mukavat.

Because the subject is plural.

• on = is for singular
• ovat = are for plural

Since sukkahousut is plural, the verb must also be plural:

• sukkahousut ovat mukavat = the tights are comfortable

If the subject were singular, you would use on:

• Mekko on mukava. = The dress is comfortable.

Here mukavat is a predicate adjective after ovat, and it agrees with the subject sukkahousut.

So:

• mustat sukkahousut ovat mukavat = black tights are comfortable

The form mukavia is partitive plural, and it would be used in different kinds of sentences, for example when talking about an indefinite quantity or in an existential-style sentence:

• Täällä on mukavia sukkahousuja. = There are comfortable tights here.

In your sentence, the tights are the full subject, so mukavat is the natural form.

Mekon is the genitive singular of mekko (dress).

That is because alla (under/beneath) is a postposition, and postpositions usually require the noun before them to be in the genitive:

• mekon alla = under a dress / under the dress
• pöydän alla = under the table
• sängyn alla = under the bed

So mekon alla is the correct pattern.

Because alla is a postposition, not a preposition.

English says:

• under the dress

Finnish says:

• mekon alla
• literally something like dress's under

Finnish has both prepositions and postpositions, but many common location words work as postpositions.

Because Finnish does not have articles like English a/an/the.

So:

• mustat sukkahousut can mean black tights or the black tights
• mekon alla can mean under a dress or under the dress

The exact meaning depends on context. Finnish leaves that unstated unless something else in the sentence makes it specific.

Yes. Finnish word order is fairly flexible.

The sentence starts with Talvella to set the time frame first: In winter...

That is very natural if the speaker is talking about seasonal clothing. You could also say something like:

• Mustat sukkahousut ovat talvella mukavat mekon alla.

This is still understandable, but the emphasis changes a bit. The original version sounds natural because it introduces the situation first: in winter.

Here it most naturally means comfortable or pleasant to wear.

So the sentence is about how the tights feel under a dress in winter. If you wanted to emphasize warmth instead, you might choose another adjective, such as:

• lämpimät = warm

So:

• mukavat = comfortable, pleasant
• lämpimät = warm

The original sentence focuses more on comfort than on temperature.