Если Вы не видите галочку, проверьте, правильно ли заполнено это поле.

Capitalized Вы is the polite/formal you in writing. Russian often uses lowercase вы, but uppercase Вы can show extra politeness, especially in instructions, messages to users, letters, or customer-facing text.

So here it sounds like a polite instruction addressed to the user.

Because галочку is the accusative singular form.

The verb видеть takes a direct object:

• видеть что? = to see what?

So:

• nominative: галочка
• accusative: галочку

This is normal for feminine nouns ending in -а / -я.

In everyday Russian, галочка often means a check mark / tick.

In interface language, it can refer to:

• the check mark itself
• sometimes a checked option
• sometimes, loosely, a checkbox

It is a very common word in apps, forms, and settings.

Проверьте is the imperative form of проверить: check.

It is:

• plural or formal singular
• from the perfective verb проверить

Why perfective? Because the instruction is about a complete action:

• check it
• not be checking / check repeatedly

So проверьте means something like check / make sure.

They do different jobs.

• если = if in a real condition

  • Если Вы не видите галочку... = If you do not see the check mark...

• ли = whether / if in an indirect yes-no question

  • проверьте, правильно ли заполнено это поле = check whether this field is filled in correctly

So in English, both can sometimes be translated as if, but in Russian they are not interchangeable here.

The particle ли usually comes after the word that is being questioned or highlighted.

Here:

• правильно ли заполнено это поле

This puts focus on правильно:

• whether it is filled in correctly

In other words, the question is not mainly whether the field is filled in at all, but whether it is filled in correctly.

Russian word order with ли is flexible, but placement affects emphasis.

Заполнено is a short-form passive participle, neuter singular, from заполнить.

It means:

• filled in
• has been filled in

It agrees with поле, which is a neuter noun:

• поле → заполнено

So:

• это поле заполнено = this field is filled in / has been filled in

This is a very common structure in Russian, especially in formal or technical language.

Because это поле is the grammatical subject of заполнено.

The structure is basically:

• это поле заполнено = this field is filled in

So поле is not the direct object of check.
The direct object of проверьте is really the whole idea: check whether this field is filled in correctly.

That is why это поле stays in the nominative.

Yes, that would also be grammatical, but the emphasis is a little different.

Compare:

• правильно ли заполнено это поле
  = emphasis on correctly
• заполнено ли это поле правильно
  = a bit more neutral, but can sound slightly less natural in this context

The original version is very natural in instructions because the key issue is correctness.

Because Russian often uses the present tense in instructions for a real current situation:

• Если Вы не видите галочку...
  = If you do not see the check mark...

This refers to what is happening now on the screen or in the form. English does the same:

• If you don’t see it, check...

So this is completely normal.

It is fairly formal / polite / instructional.

Signs of that:

• Вы instead of ты
• проверьте as a polite instruction
• technical/formal wording like заполнено это поле

This sounds like something you might see in:

• a website
• an app
• a support message
• a form instruction

If it were informal, it might use ты forms instead.