Перед отправкой нужно поставить галочку в этом поле.

Because перед here means before in a time sense, and with that meaning it normally takes the instrumental case.

• отправка = sending, submission
• instrumental singular: отправкой

So перед отправкой literally means before the sending / before submission.

This is a very common pattern:

• перед уроком = before the lesson
• перед встречей = before the meeting
• перед отъездом = before leaving

Нужно is an impersonal word meaning it is necessary, one must, or you need to, depending on context.

In this sentence, there is no explicit subject like you. Russian often leaves that unstated when the meaning is general or obvious from context.

So:

• нужно поставить галочку = it is necessary to tick/check the box
• more naturally in English: you need to tick the box

Поставить is the perfective infinitive, while ставить is imperfective.

Here, perfective is used because the sentence is about completing one specific action before another action happens: first you tick the box, then you submit.

So:

• нужно поставить галочку = you need to tick it successfully / do it once
• нужно ставить галочку would sound more like a repeated or habitual action, such as you need to tick a box every time

Because галочку is the accusative singular form, and it is the direct object of поставить.

The dictionary form is:

• галочка = a tick, check mark

After a transitive verb like поставить, you need the accusative:

• поставить галочку = to put/place a tick

For feminine nouns ending in -а / -я, the accusative singular usually changes:

• галочка → галочку
• книга → книгу

Yes. Галочка is a diminutive form derived from галка in the sense of a check mark shape, and in modern Russian it is the normal everyday word for a tick/check mark in a form or interface.

So поставить галочку is a very common fixed expression meaning:

• to tick a box
• to check a checkbox

Even though it is formally a diminutive, native speakers usually do not feel it as especially cute or emotional in this context.

Yes, поле literally means field, but it is also used for a field in a form, interface, or document.

In this sentence, в этом поле means something like:

• in this field
• in this box
• in this area of the form

Depending on context, it may refer to a form field or the area where the checkbox appears.

Because after в meaning in, when you are talking about location, Russian uses the prepositional case.

The noun is:

• поле

Its prepositional singular is:

• в поле

And the demonstrative adjective must match it:

• это поле
• в этом поле

So в этом поле means in this field / in this box.

Russian often leaves out the subject when it is general, obvious, or not important.

This sentence is an impersonal instruction:

• Перед отправкой нужно поставить галочку в этом поле.

It has the feel of a notice, interface instruction, or rule. English often supplies you, but Russian does not need to.

A fuller version could be:

• Перед отправкой вам нужно поставить галочку в этом поле.
• Перед отправкой нужно, чтобы вы поставили галочку в этом поле.

But the original is more natural for a general instruction.

Yes. Russian word order is flexible, and changing it usually changes emphasis rather than basic meaning.

For example:

• Перед отправкой нужно поставить галочку в этом поле.
• Нужно поставить галочку в этом поле перед отправкой.

Both are grammatical. Starting with Перед отправкой highlights the time condition first: before submitting...

That is very natural in instructions.

Yes. That is another very common way to say before sending/submitting.

Compare:

• Перед отправкой нужно поставить галочку в этом поле.
• Перед тем как отправить, нужно поставить галочку в этом поле.

The first version is more compact and slightly more formal or instruction-like.
The second feels a bit more verbal and conversational.

Both are correct.

Grammatically, отправка means sending. But in actual context, especially on websites or forms, it often corresponds to submission.

So depending on what is being sent, English might translate it as:

• before sending
• before submitting
• before you submit

This is more about context and natural English wording than about a different Russian grammar point.