Ova vežba je sasvim laka.

Because ova must agree with the noun vežba.

• vežba is feminine singular
• so the demonstrative this must also be feminine singular

• the forms are:

• ovaj = this, masculine
• ova = this, feminine
• ovo = this, neuter

So:

• ovaj zadatak = this task
• ova vežba = this exercise
• ovo pitanje = this question

In your sentence, vežba is feminine, so ova is the correct form.

For the same reason: it agrees with vežba, which is feminine singular.

The adjective lak means easy/light, and its basic gender forms are:

• lak = masculine
• laka = feminine
• lako = neuter

So:

• zadatak je lak = the task is easy
• vežba je laka = the exercise is easy
• pitanje je lako = the question is easy

Even though laka comes after je, it still agrees with the subject vežba.

It is nominative singular.

Why?

• vežba is the subject of the sentence
• subjects are normally in the nominative
• ova matches it in gender, number, and case

So:

• ova vežba = nominative singular feminine

The adjective laka is also in the nominative singular feminine, because with biti (to be), Serbian uses a predicate adjective that agrees with the subject.

sasvim usually means something like:

• quite
• completely
• entirely

In this sentence, the most natural English feeling is probably quite or perfectly:

• Ova vežba je sasvim laka. = This exercise is quite easy.

The exact strength depends on context. sasvim can sometimes sound a bit stronger than English quite. It often suggests that something is fully or definitely the case.

Compare:

• laka = easy
• vrlo laka = very easy
• sasvim laka = quite / completely easy

je is the 3rd person singular present form of biti (to be).

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

Since vežba is singular, Serbian uses je:

• vežba je laka = the exercise is easy

Unlike some languages, Serbian normally does use the present-tense form of to be in sentences like this.

Normally, no in standard Serbian.

You should say:

• Ova vežba je laka.

Not:

• Ova vežba laka.

Omitting je may happen in special styles such as headlines, notes, or poetry, but for normal speech and writing, keep it.

The sentence you have is the most neutral and natural order:

• Ova vežba je sasvim laka.

Serbian word order is fairly flexible, but changes can affect emphasis.

For example:

• Ova vežba je laka. = neutral
• Ova vežba je sasvim laka. = neutral, with sasvim modifying laka
• Sasvim je laka ova vežba. = possible, but more marked or emphatic
• Laka je ova vežba. = emphasis on easy

So the given order is a good standard pattern for learners.

Yes, vežba is a feminine noun.

A good clue is the ending -a, which often marks feminine nouns in Serbian.

That is why you get:

• ova vežba
• teška vežba
• dobra vežba

All the words describing it must match the feminine gender.

A rough pronunciation is:

• VEZH-ba

A few helpful points:

• ž sounds like the s in measure or vision
• v is like English v
• e is a clear e sound, like in bet but steadier
• b is a normal b
• a is like a in father

So vežba sounds approximately like VEZH-bah.

Not exactly.

• vrlo laka = very easy
• sasvim laka = quite easy, perfectly easy, or completely easy, depending on context

So vrlo focuses more on degree: very.
sasvim often suggests that there is no real difficulty at all, or that something is fully easy.

In many everyday contexts, both can be translated naturally, but the nuance is a little different.

Yes, you can.

• Ova vežba je sasvim laka.
• Ova je vežba sasvim laka.

Both are grammatical.

The second version puts a little more focus on ova or sounds slightly more structured, but the first one is the more neutral default pattern for learners.