Proveravam dasku libelom i merim je mernom trakom.

Because dasku is in the accusative case, which marks the direct object of the verb.

In this sentence, the thing being checked is the board/plank, so Serbian uses the accusative:

• daska = board, plank (dictionary form / nominative)
• dasku = board, plank (accusative singular)

So:

• Proveravam dasku = I am checking the board

This is very common with feminine nouns ending in -a:

• nominative: kuća
• accusative: kuću

The same pattern gives:

• daska → dasku

They are in the instrumental case, which is often used to show the tool, means, or instrument used to do something.

So here:

• libelom = with a level
• mernom trakom = with a measuring tape

The sentence is saying:

• I check the board using a level
• I measure it using a measuring tape

For feminine singular nouns, the instrumental often ends in -om:

• libela → libelom
• traka → trakom

And the adjective has to match the noun:

• merna traka = measuring tape
• mernom trakom = with a measuring tape

No. In this sentence, Serbian does not need a separate word for with because the instrumental case itself already expresses that idea.

So English says:

• with a level
• with a measuring tape

But Serbian can simply say:

• libelom
• mernom trakom

Sometimes Serbian does use sa / s for with, but that is not necessary here when the instrumental alone naturally expresses the tool being used.

Here je means it, not is.

It is the unstressed accusative pronoun for a feminine singular noun. It refers back to dasku.

So:

• dasku = the board
• merim je = I measure it

This is similar to English avoiding repetition:

• Proveravam dasku...
• ...i merim je...

instead of repeating dasku again.

Important distinction:

• je can also mean is in other sentences
• here, it is a pronoun, not a form of to be

Because je is the normal clitic (unstressed short form) of the feminine accusative pronoun, while nju is the stressed full form.

So:

• merim je = I measure it
• nju merim = I measure her/it (more emphatic, contrastive, or used in special positions)

A learner can usually think of it this way:

• je = ordinary it
• nju = it/her with emphasis, contrast, or after prepositions

Examples:

• Merim je. = I’m measuring it.
• Merim nju, ne onu drugu. = I’m measuring that one, not the other one.

Because Serbian clitic pronouns like je usually have special placement rules. They tend to go in the second position of the clause, or immediately after the first stressed element.

In the second part of the sentence:

• i merim je mernom trakom

the first main word of that clause is merim, so the clitic je comes right after it:

• merim je

This is very natural Serbian word order.

You may also see other word orders in Serbian, but clitics like je are not placed completely freely. For a learner, merim je is the safe and standard pattern.

Serbian present tense is often used for:

• something happening right now
• a habitual action
• a description of what someone does in a process

So this sentence can mean something like:

• I’m checking the board with a level and measuring it with a measuring tape
• or I check the board with a level and measure it with a measuring tape

Serbian present tense often covers both English simple present and present continuous, depending on context.

Because proveravam is imperfective, and that fits an action viewed as ongoing, repeated, or in progress.

Very roughly:

• proveravati / proveravam = to be checking, to check regularly
• proveriti / proverim = to check and complete the check

In a sentence describing a process or ongoing action, Serbian often prefers the imperfective:

• Proveravam dasku... = I’m checking the board...

If you used the perfective form, it would usually point more to a completed single action, and perfective verbs normally do not describe an action happening right now in the ordinary present tense.

Yes. Merim is the imperfective verb meriti.

A common perfective partner is izmeriti:

• meriti / merim = to measure, be measuring
• izmeriti / izmerim = to measure completely, get the measurement

So:

• Merim je mernom trakom. = I’m measuring it with a measuring tape.
• Izmeriću je. = I will measure it / I will take the measurement.

Again, the imperfective fits well in a sentence describing what someone is doing as part of a task.

Because in Serbian, adjectives agree with nouns in gender, number, and case.

The base phrase is:

• merna traka = measuring tape

Here:

• merna is the adjective
• traka is the noun

Since the whole phrase is in the instrumental singular feminine, both words change:

• merna traka
• mernom trakom

This agreement happens throughout Serbian:

• dobra knjiga

• dobrom knjigom

• nova kuća
• novu kuću

So mernom trakom is exactly what you should expect.

Libela means a level, especially a spirit level used to check whether something is horizontal or vertical.

Depending on region or context, people may also use other terms, but libela is a normal and useful word to know, especially in practical, building, or workshop contexts.

So in this sentence:

• libelom = with a spirit level

Yes, you can add ja, but it is usually omitted unless you want emphasis or contrast.

Serbian verb endings already show the subject:

• proveravam = I am checking
• merim = I am measuring

So the sentence naturally appears without ja:

• Proveravam dasku libelom i merim je mernom trakom.

If you add ja, it sounds more emphatic:

• Ja proveravam dasku libelom i merim je mernom trakom.

That could mean something like:

• I’m the one checking the board and measuring it

So omission is normal; adding the pronoun adds emphasis.

Yes, it could, but using je sounds more natural because it avoids repetition.

Compare:

• Proveravam dasku libelom i merim je mernom trakom.
  = natural: I check the board with a level and measure it with a measuring tape.

• Proveravam dasku libelom i merim dasku mernom trakom.
  = grammatical, but repetitive

So Serbian behaves much like English here: once the object is clear, a pronoun is often preferred.