Мама ровно заплела дочке косу.

Because ровно is an adverb here: it describes how the action was done.

• ровно заплела = braided neatly / evenly / smoothly
• It modifies the verb заплела

If you used an adjective like ровную, it would have to describe a noun, for example:

• ровную косу = a neat/even braid

So in this sentence, ровно tells us about the manner of braiding, not about the noun by itself.

Also, ровно can mean exactly in other contexts, but here it means something like evenly or neatly.

дочке is in the dative singular because it marks the person for whom or to whom the action is done.

Here the structure is basically:

• Мама заплела дочке косу
  = Mom braided her daughter’s hair into a braid / braided her daughter a braid

Russian often uses the dative with body-related or personal-care actions when something is done to someone or for someone:

• Мама помыла сыну голову. = Mom washed her son’s hair/head.
• Бабушка связала внуку шарф. = Grandma knitted her grandson a scarf.

So дочке is not the direct object. It is the recipient/affected person.

Because косу is the direct object: it is the thing that was braided.

The base form is:

• коса = braid / plait

In the accusative singular, it becomes:

• косу

This is normal for a feminine noun ending in -а:

• nominative: коса
• accusative: косу

So:

• дочке = the person affected
• косу = the thing produced/formed by the action

That is just a natural Russian pattern.

With заплести, Russian often says:

• заплести кому-то косу
• literally: to braid someone a braid

English usually prefers something like:

• to braid someone’s hair
• to do someone’s hair in a braid
• to braid her daughter’s hair

Russian does not need to mention волосы here, because коса already makes the hair meaning clear.

So this sentence sounds natural in Russian, even if the literal structure feels unusual to an English speaker.

Because заплела is perfective, while плела is imperfective.

• плела = was braiding / used to braid / braided in a process sense
• заплела = braided it successfully to completion

In this sentence, the action is presented as completed: the mother finished braiding the braid.

That is why заплела fits better.

The prefix за- here helps form the perfective verb заплести, which commonly means to braid up / to braid into shape / to complete the braid.

Because Russian past tense agrees with the subject in gender and number.

The subject is Мама, which is feminine singular, so the verb is feminine singular too:

• masculine: заплёл
• feminine: заплела
• neuter: заплело
• plural: заплели

So заплела tells you that the person doing the action is feminine.

Yes, Russian word order is relatively flexible.

The sentence:

• Мама ровно заплела дочке косу.

is perfectly natural and fairly neutral.

But other orders are also possible, with different emphasis:

• Мама дочке ровно заплела косу.
• Дочке мама ровно заплела косу.
• Косу мама дочке заплела ровно.

The basic meaning stays the same because the case endings show the grammatical roles.

In the original sentence:

• Мама comes first as the subject
• ровно appears before the verb and sounds natural as the manner adverb
• дочке косу comes after the verb in a normal, clear order

So the original word order is a good neutral model for learners.

No. Коса can have more than one meaning in Russian, depending on context.

Common meanings include:

• braid / plait
• scythe

In this sentence, it clearly means braid, because of the verb заплела and the context with дочке.

So this is one of those words where context does the work.

Because Russian has no articles, and possession is often left to context.

So:

• Мама can mean Mom, the mother, or sometimes my mom, depending on context.
• дочке can mean to the daughter, to her daughter, to his daughter, etc.
• косу can mean a braid or the braid, depending on the situation.

Russian speakers usually understand these details from context rather than from articles like the or a.

That means when translating into English, you often have to add information that Russian leaves unstated.