Mereka mengisytiharkan aktiviti perniagaan itu.

Yes, mereka means they / them and refers to more than one person.

A few useful notes:

• It is not marked for gender, so it can mean they for men, women, or a mixed group.
• It usually refers to people, not things.
• Unlike English, Malay does not change the form between subject and object, so mereka can mean both they and them depending on the sentence.

In this sentence, mereka is the subject: they.

Because it contains a root word plus affixes.

Here is the breakdown:

• isytihar = declare / proclamation
• meng- = a verbal prefix that often forms an active verb
• -kan = a suffix that often makes the verb transitive or gives it a more object-taking meaning

So:

• isytihar → root
• mengisytiharkan → to declare / to proclaim something

In this sentence, the verb needs an object, and that object is aktiviti perniagaan itu.

Because isytihar by itself is the base form, while mengisytiharkan is the fully inflected verb used in a normal active sentence.

Malay often uses:

• a base/root form in dictionary-style citation
• an affixed form in actual sentences

Compare:

• isytihar = declare (base idea)
• mengisytiharkan = declared / declare / is declaring, depending on context

So in a complete sentence with a subject doing the action, mengisytiharkan is the natural form.

It is a noun phrase meaning business activity.

Breakdown:

• aktiviti = activity
• perniagaan = business / التجارة-like commercial activity

In Malay, nouns often combine directly like this:

• aktiviti perniagaan = business activity
• literally something like activity of business

The second noun often functions like an English modifier:

• kad kredit = credit card
• pusat bandar = city centre
• aktiviti perniagaan = business activity

Because in Malay, demonstratives such as ini and itu usually come after the noun phrase.

So:

• rumah itu = that house / the house
• aktiviti perniagaan itu = that business activity / the business activity

This is very different from English, where this/that usually comes before the noun.

Also notice that itu comes after the whole noun phrase, not just after aktiviti:

• aktiviti perniagaan itu not
• itu aktiviti perniagaan in this kind of normal noun phrase

It can feel like either one in translation, depending on context.

In Malay, itu literally means that, but in many sentences it also helps make the noun phrase definite, so in natural English it may be translated as:

• that business activity or
• the business activity

Malay does not have articles exactly like the and a/an, so words like itu often help show specificity.

The sentence follows the common Malay pattern:

Subject + Verb + Object

So:

• Mereka = subject
• mengisytiharkan = verb
• aktiviti perniagaan itu = object

That makes the structure very similar to English:

• They declared that business activity.

So although some details differ, the core word order here is quite familiar to an English speaker.

Malay verbs usually do not change form for tense the way English verbs do.

So mengisytiharkan by itself does not tell you whether the action is:

• present
• past
• future

You usually know the time from:

1. context
2. time words, such as:
   • semalam = yesterday
   • hari ini = today
   • esok = tomorrow
3. aspect markers like:
   • sudah / telah = already
   • sedang = in progress
   • akan = will

So this sentence could mean:

• They declare that business activity
• They declared that business activity
• They have declared that business activity

The correct English translation depends on context.

Yes, it sounds fairly formal, mainly because of the verb mengisytiharkan.

isytihar / mengisytiharkan is a word often used in:

• official statements
• legal contexts
• government announcements
• public declarations

In more everyday speech, people might sometimes choose other verbs depending on the intended meaning, such as:

• mengumumkan = announce
• menyatakan = state
• memberitahu = inform / tell

So this sentence sounds natural, but not especially casual.

Because Malay does not use articles the same way English does.

English requires choices like:

• a business activity
• the business activity

Malay usually leaves that unstated unless context or another word makes it clear.

Here, itu helps make the noun phrase specific, so aktiviti perniagaan itu is understood as something like:

• that business activity
• the business activity

Without itu, aktiviti perniagaan would be less specific.

Yes. A natural passive version would be:

Aktiviti perniagaan itu diisytiharkan oleh mereka.

Breakdown:

• Aktiviti perniagaan itu = the business activity
• diisytiharkan = was declared / is declared
• oleh mereka = by them

This is useful because Malay uses both active and passive forms quite a lot.

Compare:

• Mereka mengisytiharkan aktiviti perniagaan itu. = They declared the business activity.
• Aktiviti perniagaan itu diisytiharkan oleh mereka. = The business activity was declared by them.

Malay nouns often do not have to change form for singular vs. plural.

So aktiviti can sometimes mean:

• activity or
• activities

The context tells you which meaning is intended.

If Malay wants to make plurality especially clear, it can use:

• reduplication: aktiviti-aktiviti
• number words
• context

In this sentence, aktiviti perniagaan itu most naturally sounds like that business activity or the business activity, but in some contexts a broader plural interpretation could be possible.

A rough learner-friendly guide is:

meng-isytihar-kan

Useful pronunciation notes:

• meng sounds roughly like muhng or meng
• sy in isytihar sounds like sh
• kh is not present here; the sound is simpler than some learners expect
• r in Malay is usually tapped or lightly rolled, not the strong English r

A rough approximation: meng-ish-ti-har-kan

If you speak carefully in syllables, you will usually be understood well.