Sumnjam da je akumulator siguran.

Sumnjam is the 1st person singular present tense of the verb sumnjati, which means to doubt or to suspect.

So:

• sumnjam = I doubt / I suspect
• dictionary form: sumnjati

In this sentence, it means I doubt.

Da introduces a subordinate clause, very often corresponding to English that.

So:

• Sumnjam da... = I doubt that...

In Serbian, this is a very common structure after verbs of thinking, saying, knowing, doubting, and similar ideas.

Examples:

• Mislim da je u pravu. = I think that he is right.
• Znam da dolazi. = I know that he is coming.
• Sumnjam da je akumulator siguran. = I doubt that the battery is safe.

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

Here it means is:

• da je akumulator siguran = that the battery is safe

So the clause breaks down like this:

• da = that
• je = is
• akumulator = the battery
• siguran = safe / secure

This is because je is a clitic in Serbian. Clitics are short, unstressed words that usually appear in a fixed early position in the clause, often second position.

After da, the clitic je comes very early:

• da je akumulator siguran = correct
• da akumulator je siguran = not the normal standard order

This word order can feel unusual to English speakers, because English keeps the subject before is, but Serbian clitic placement follows its own rules.

Akumulator is in the nominative singular because it is the subject of the subordinate clause.

In da je akumulator siguran, the thing that is safe is akumulator, so it stays in nominative.

Also, akumulator is a masculine noun, which matters for adjective agreement.

Because adjectives in Serbian must agree with the noun they describe in gender, number, and case.

Here:

• akumulator is masculine singular nominative
• so the adjective must also be masculine singular nominative
• that form is siguran

Comparison:

• siguran = masculine singular
• sigurna = feminine singular
• sigurno = neuter singular

Examples:

• telefon je siguran = the phone is safe/secure
• baterija je sigurna = the battery is safe/secure
• mesto je sigurno = the place is safe

Siguran can mean several related things depending on context, such as:

• safe
• secure
• certain / sure
• sometimes reliable

In this sentence, because it describes akumulator, it most naturally means something like safe or secure.

A learner should know that siguran is a broad adjective, and the exact English translation depends on context.

Not usually in everyday use. In Serbian, akumulator most commonly means a rechargeable battery, especially a car battery.

So in many contexts:

• akumulator = car battery / storage battery

An English speaker may recognize the word, but in Serbian it is much more common in everyday speech than accumulator is in modern everyday English.

Grammatically, yes. The sentence is correct.

However, depending on the situation, a Serbian speaker might choose a different adjective if they mean something more specific.

For example:

• siguran = safe / secure
• bezbedan = safe, often with a stronger physical-safety sense
• ispravan = functioning properly / in good working order

So if the intended meaning is I doubt the battery is safe, the sentence works.
If the intended meaning is I doubt the battery is working properly, then ispravan might be more natural.

Yes. Sumnjati can be used in more than one pattern.

Two common ones are:

1. sumnjati da... = to doubt that...

   • Sumnjam da je tačno. = I doubt that it is true.

2. sumnjati u + accusative = to doubt / have doubts about

   • Sumnjam u njega. = I doubt him / I have doubts about him.
   • Sumnjam u tu priču. = I doubt that story.

So in your sentence, Serbian uses the da-clause because a whole statement follows.

If you say:

• Ne sumnjam da je akumulator siguran.

that means:

• I do not doubt that the battery is safe.

This is important because English and Serbian work similarly here: negating sumnjam changes the meaning a lot.

Compare:

• Sumnjam da je akumulator siguran. = I doubt that the battery is safe.
• Ne sumnjam da je akumulator siguran. = I do not doubt that the battery is safe.

Yes, but the nuance changes.

Compare these:

• Sumnjam da je akumulator siguran.
  = I doubt that the battery is safe.

• Sumnjam da akumulator nije siguran.
  = I suspect / doubt that the battery is not safe.

The first one expresses doubt about the positive statement it is safe.
The second one introduces explicit negation inside the subordinate clause.

English can also do something similar, but Serbian learners should be careful, because small changes in negation can shift the meaning.

Here is the structure:

• Sumnjam — verb, 1st person singular present, I doubt
• da — conjunction, that
• je — auxiliary/copular form of biti, is
• akumulator — noun, masculine singular nominative, battery / car battery
• siguran — adjective, masculine singular nominative, agreeing with akumulator, safe / secure

So the sentence literally maps as:

• I doubt that is battery safe

but in natural English:

• I doubt that the battery is safe.