Sada razumijemo rebus.

Because the ending -mo marks 1st person plural in the present tense, so razumijemo means we understand.

The verb here comes from razumjeti = to understand.

A very simple breakdown is:

• ja razumijem = I understand
• ti razumiješ = you understand
• on/ona/ono razumije = he/she/it understands
• mi razumijemo = we understand
• vi razumijete = you all / you formal understand
• oni/one/ona razumiju = they understand

So even without the pronoun mi, the form razumijemo already tells you the subject is we.

In Croatian, subject pronouns are often omitted when they are clear from the verb form.

So:

• Sada razumijemo rebus.
• Mi sada razumijemo rebus.

Both mean the same thing: Now we understand the rebus.

The version without mi is very normal and natural, because razumijemo already shows that the subject is we.

You would add mi only if you want emphasis, contrast, or clarity:

• Mi sada razumijemo rebus, a oni ne.
  We understand the rebus now, but they do not.

Sada means now.

Yes, in everyday Croatian, sad is also very common and usually means the same thing.

So both of these are natural:

• Sada razumijemo rebus.
• Sad razumijemo rebus.

Sada can feel slightly more neutral or a bit more formal, while sad is very common in speech.

Because rebus is the direct object of the verb razumijemo, so it is in the accusative case.

However, rebus is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: rebus
• accusative: rebus

That is why the form does not change here.

Compare this with a masculine animate noun, where accusative usually does change:

• nominative: student
• accusative: studenta

So:

• Vidim studenta. = I see the student.
• Razumijemo rebus. = We understand the rebus.

Croatian does not have articles like English a and the.

So rebus can mean:

• a rebus
• the rebus

The exact meaning depends on context.

In your sentence, English may translate it as the rebus because the situation probably makes it clear which rebus is meant. But Croatian itself does not add a separate word like the.

Yes. Croatian word order is more flexible than English word order.

Sada razumijemo rebus. is a very natural order and puts now near the front for emphasis on time.

Other possible orders include:

• Razumijemo rebus sada.
• Rebus sada razumijemo.
• Mi sada razumijemo rebus.

But they do not all sound equally neutral.

The most natural neutral version here is:

• Sada razumijemo rebus.

Croatian often changes word order for emphasis, rhythm, or style, not just grammar.

A helpful approximate pronunciation is:

ra-zu-MI-je-mo

The stress can vary somewhat depending on dialect and teaching style, but the important thing for a learner is to hear the parts clearly:

• ra
• zu
• mi
• je
• mo

A few pronunciation notes:

• r is rolled or tapped.
• j is pronounced like English y in yes.
• So ije sounds roughly like ee-yeh.

That means razumijemo is not pronounced like one English-style long vowel. You should clearly hear the j sound.

For a learner, the main thing to notice is that razumjeti belongs to a group of verbs whose present-tense forms contain -ije- / -ij- patterns:

• razumijem
• razumiješ
• razumije
• razumijemo

So the form razumijemo may look longer than you expect if you are trying to build it directly from the infinitive, but it is a normal present-tense form.

It is best to learn the present forms as a set rather than trying to guess them from English.

Yes, that can also work, depending on context.

Both razumjeti and shvatiti/shvaćati can relate to understanding, but they are not always used in exactly the same way.

• razumjeti often means to understand
• shvatiti often means to grasp / realize / figure out
• shvaćati is the imperfective partner often used for ongoing or repeated understanding/grasping

In a sentence like this, Sada razumijemo rebus sounds very natural for Now we understand the rebus.

If the idea is more like Now we’ve finally figured it out, Croatian might also use a form related to shvatiti.

Yes. Sada razumijemo rebus. is a complete, natural sentence.

It has:

• a time adverb: sada = now
• a verb: razumijemo = we understand
• a direct object: rebus

So grammatically it is fully complete.

It could easily appear in a conversation like this:

• Prije nam nije bio jasan. = Before, it wasn’t clear to us.
• Sada razumijemo rebus. = Now we understand the rebus.

Rebus is a masculine noun.

For practical purposes in this sentence, the key points are:

• it is singular
• it is masculine
• it is inanimate
• in the accusative singular, it stays the same as the nominative

So:

• To je rebus. = That is a rebus.
• Razumijemo rebus. = We understand the rebus.

If you continue learning the noun, you will eventually meet other case forms, but in this sentence the unchanged form rebus is exactly what you should expect.