Сначала заполните бланк, а потом поставьте галочку.

Because they are imperative forms, meaning they give an instruction: fill out and put/check.

The ending -ите here can do two things:

• mark a command to more than one person
• or make the command polite/formal when speaking to one person

So:

• заполни = fill out! (informal, to one person)
• заполните = fill out! (formal to one person, or to several people)

Likewise:

• поставь = put!/mark!
• поставьте = put!/mark! (formal/plural)

This sentence sounds like an instruction on a form, in an office, or from a staff member, so the polite/formal imperative is natural.

Сначала means first / at first, and потом means then / after that.

The pattern сначала ..., а потом ... is a very common way to sequence actions:

• First do X, and then do Y.

The word а here does not strongly mean but. In many contexts, а simply links two parts while contrasting or separating them a little:

• first one action,
• then another action.

So the whole structure means:

• First fill out the form, and then check the box.

You could think of а потом here as something like and then / and after that.

Because бланк is the direct object of заполните, and it is an inanimate masculine noun.

For inanimate masculine nouns in the singular, the accusative usually looks the same as the nominative:

• nominative: бланк
• accusative: бланк

So заполните бланк means fill out the form.

If it were a different noun type, the form might change. For example:

• заполните анкету = fill out the questionnaire
  Here анкету is feminine accusative singular.

Because галочку is the accusative singular form of галочка.

The verb поставить takes a direct object: you put/place/mark something. Here, what you mark is галочку.

This noun is:

• feminine
• singular
• animate? no, inanimate
• direct object, so accusative

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

• галочка → галочку

Compare:

• книга → книгу
• ручка → ручку

So:

• поставьте галочку = put a check mark / tick the box

Russian often expresses this idea with the phrase поставить галочку, literally to put a check mark.

This is just the normal idiomatic way to say:

• check the box
• tick the box
• mark with a check

A few useful related expressions are:

• поставить галочку = to put a check mark / tick
• отметить = to mark, indicate
• сделать отметку = to make a mark

So even though English often uses one verb, check, Russian commonly uses a verb plus noun combination.

Yes. Галочка is the diminutive form of галка in the meaning of a check mark / tick.

In practice, галочка is the normal everyday word for the symbol ✓. Even though it is historically a diminutive-looking form, speakers usually just treat it as the standard word in this context.

So:

• поставить галочку = put a tick/check mark
• поставить крестик = put an X

English learners sometimes expect a more literal word for checkmark, but галочка is the natural choice.

Because the sentence is telling someone to complete two single, finished actions.

Perfective verbs focus on:

• completion
• result
• doing the action once

Here the speaker wants the result:

1. the form gets filled out
2. the check mark gets placed

That is why Russian uses:

• заполнить → заполните
• поставить → поставьте

If the sentence were about process, repeated action, or general instruction in a different tone, imperfective forms might appear in other contexts. But for practical step-by-step instructions, perfective imperatives are very common.

Usually no, not with this meaning.

• заполнить бланк = fill out a form
• записать = write down / record

So:

• заполните бланк means complete all the necessary fields on the form
• запишите would suggest writing something down, not completing the whole form as a form

For forms, applications, questionnaires, etc., заполнить is the standard verb.

Not exactly, though they can overlap in translation.

• бланк usually means a blank form, a printed document you fill in
• форма can mean form too, especially in modern contexts, but it is a broader word and can also mean shape, uniform, or grammatical form

In office or bureaucratic language, бланк is very common for a physical form/document.

So in this sentence:

• заполните бланк sounds very natural for fill out the form

Yes, but the given order is the most straightforward for instructions.

Current version:

• Сначала заполните бланк, а потом поставьте галочку.

This is clear and natural because it mirrors the order of actions.

Russian word order is flexible, so other versions are possible, for example:

• Заполните сначала бланк, а потом поставьте галочку.

That still means basically the same thing, but it shifts the emphasis slightly.

For a learner, the important point is:

• сначала marks the first action
• потом marks the next action
• Russian does not depend on rigid word order as much as English does

It can mean both, depending on context.

Most often, especially with forms, it means:

• check the box
• tick the appropriate option

But literally it just means:

• put a check mark

So the exact place is understood from the situation. On a form, that usually means in a box or next to an option.

If Russian wants to be more explicit, it can say things like:

• поставьте галочку в нужном поле = put a check mark in the correct field
• поставьте галочку напротив нужного пункта = put a check mark next to the correct item