В личном кабинете нужно заполнить одно поле и поставить галочку.

Not literally. In modern Russian, личный кабинет often means a personal account, user account, or account dashboard on a website or app.

So:

• в личном кабинете = in your account / in the personal account section

Grammatically:

• в = in
• личном кабинете is in the prepositional case because it follows в in a location meaning.

Dictionary form:

• личный кабинет
• prepositional: в личном кабинете

This is a very common phrase in online services.

Because this sentence is talking about location, not motion.

Russian uses:

• в + accusative for motion into something
  
  • войти в личный кабинет = to enter the personal account
• в + prepositional for being inside / within something
  
  • в личном кабинете нужно... = in the personal account, it is necessary to...

So here, the meaning is within the account interface, not go into the account.

Нужно means something like:

• it is necessary
• one needs to
• you need to

This is an impersonal construction, which is very common in Russian.

So:

• нужно заполнить... = it is necessary to fill in...
• in natural English: you need to fill in...

Russian often leaves out вам if it is obvious from context. You could say:

• В личном кабинете вам нужно заполнить...

But without вам, the sentence still sounds natural and standard, especially in instructions.

Because they depend on нужно.

Pattern:

• нужно + infinitive = need to + verb

So:

• нужно заполнить = need to fill in
• нужно поставить = need to put / place / tick

This is one of the most useful Russian patterns for giving instructions:

• Нужно выбрать язык. = You need to choose a language.
• Нужно нажать кнопку. = You need to press the button.

They are perfective because the sentence is about completing specific actions.

• заполнять / ставить = imperfective, process or repeated action
• заполнить / поставить = perfective, one completed action

Here the instruction is not about the process in general, but about what the user must do once:

• fill in one field
• tick a checkbox

So perfective is the most natural choice.

If you used the imperfective, it would sound less like a concrete instruction and more like a general activity or ongoing process.

Одно поле is accusative singular, but because поле is a neuter inanimate noun, its accusative looks exactly like the nominative.

Dictionary form:

• поле = field

Forms:

• nominative: одно поле
• accusative: одно поле

It is accusative here because it is the direct object of заполнить:

• заполнить что? → одно поле

Also notice the numeral/adjective agreement:

• один with masculine nouns
• одна with feminine nouns
• одно with neuter nouns

Since поле is neuter, we get одно поле.

Yes, in website or form language, заполнить поле means:

• fill in a field
• complete a field
• sometimes simply enter information in a field

It is the normal Russian verb for filling in forms:

• заполнить анкету = fill in a form/questionnaire
• заполнить поле = fill in a field

So this is standard interface/instruction vocabulary.

Поставить галочку is a very common expression meaning:

• tick a box
• check a checkbox
• mark a checkbox

Literally:

• поставить = to put
• галочка = little check mark

Russian often uses this phrase for checkboxes in forms and settings.

Examples:

• Поставьте галочку рядом с пунктом... = Tick the box next to the item...
• Нужно поставить галочку, если вы согласны. = You need to check the box if you agree.

So yes, the literal image is put a checkmark, but idiomatically it just means check the box.

Because it is the direct object of поставить, so it is in the accusative case.

Dictionary form:

• галочка (nominative singular)

Accusative singular:

• галочку

This happens because галочка is a feminine noun ending in -а:

• nominative: галочка
• accusative: галочку

Compare:

• читать книгу
• заполнить форму
• поставить галочку

Yes, галочка is a diminutive form.

The base word is:

• галка in this context = check mark

And галочка is the more common everyday form when talking about UI checkmarks.

In interface language, this diminutive does not necessarily sound especially cute or emotional. It is just the usual word many speakers use for a checkbox mark.

So learners should treat поставить галочку as a fixed, natural expression.

Because the two infinitives are coordinated with и and both depend on the same нужно.

Structure:

• нужно [заполнить одно поле] и [поставить галочку]

This is just like English:

• You need to fill in one field and tick a box.

Russian often avoids repeating words when the meaning is already clear.

You could repeat it:

• нужно заполнить одно поле и нужно поставить галочку

But that would sound heavier and less natural in a simple instruction.

Yes. Russian word order is flexible, although each version may sound slightly different in emphasis.

Original:

• В личном кабинете нужно заполнить одно поле и поставить галочку.

Possible alternatives:

• Нужно в личном кабинете заполнить одно поле и поставить галочку.
• Одно поле и галочку нужно заполнить и поставить — this one sounds awkward and unnatural
• В личном кабинете нужно поставить галочку и заполнить одно поле. — possible, but changes the order of the actions

The original version is natural because it starts with the context:

• in the personal account

and then gives the required actions.

Yes, it often does.

Depending on context, одно поле can mean:

• one field
• just one field
• a single field

In instructions, this may imply that the task is simple or minimal: you only need to fill in one field.

If the sentence wanted stronger emphasis on only, Russian might say:

• только одно поле = only one field

But even without только, одно поле can still naturally suggest a single field.

It is mostly neutral standard Russian, especially suitable for:

• website instructions
• help pages
• user guidance
• app or service interfaces

A few parts are especially typical of digital/service language:

• личный кабинет
• заполнить поле
• поставить галочку

So it sounds natural in administrative or online contexts, not overly formal and not especially casual.