ארגז הקרטון עלול ליפול אם נערום אותו גבוה מדי.

Because ארגז הקרטון is a construct chain (סמיכות) meaning the cardboard box or literally the box of the cardboard.

In this pattern, the definite article often appears on the second noun, and that makes the whole phrase definite:

• ארגז קרטון = a cardboard box
• ארגז הקרטון = the cardboard box

So Hebrew does not usually say הארגז הקרטון in this structure.

עלול means something like liable to, likely to, or may end up.

In this sentence, it suggests a risk or negative possibility:

• הארגז עלול ליפול = the box may fall / is liable to fall

This is a little different from יכול (can / is able to).
עלול often implies that something bad might happen.

Also, עלול is masculine singular, matching ארגז, which is a masculine singular noun.

After עלול, Hebrew normally uses an infinitive:

• עלול ליפול = may fall
• עלול להישבר = may break
• עלול לקרות = may happen

So עלול ליפול is a standard pattern:

• עלול = may be likely / liable
• ליפול = to fall

נערום is the 1st person plural future form of לערום (to pile up / stack).

So it means:

• נערום = we will stack / we stack

In a sentence with אם (if), Hebrew often uses the future for a possible future situation:

• אם נערום אותו... = if we stack it...

Hebrew does not need the pronoun אנחנו here, because the verb form already tells you the subject is we.

Hebrew often leaves out subject pronouns when the verb already shows the person and number.

So:

• נערום already means we will stack
• אנחנו נערום is possible, but usually unnecessary unless you want emphasis

This is very common in Hebrew.

אותו is the direct object pronoun meaning him or it for a masculine singular noun.

Here it refers back to ארגז (box), so it means it:

• נערום אותו = we stack it

Hebrew uses the same word for him and it in the masculine singular, so the context tells you which one is meant.

גבוה מדי means too high.

• גבוה = high
• מדי = too / excessively

So:

• אם נערום אותו גבוה מדי = if we stack it too high

A useful pattern is:

• גדול מדי = too big
• קטן מדי = too small
• מהר מדי = too fast
• גבוה מדי = too high

In Hebrew, מדי usually comes after the adjective or adverb it modifies.

So Hebrew says:

• גבוה מדי = too high
• יקר מדי = too expensive
• לאט מדי = too slowly

This is just the normal Hebrew word order.

Here גבוה is the natural form used with ארגז, which is masculine singular.

It helps describe the resulting position or height of the box: stack it high.
In sentences like this, Hebrew often uses the masculine singular form naturally, especially when the noun being referred to is masculine.

So אותו גבוה מדי fits well with ארגז.

Because the sentence is talking about a possible future situation:

• אם נערום אותו גבוה מדי = if we stack it too high

In Modern Hebrew, after אם, a future form is very common when the condition refers to the future.

So the structure is normal:

• אם + future verb

For example:

• אם נמהר, נספיק = if we hurry, we’ll make it
• אם תיגע בזה, זה ייפול = if you touch it, it will fall

Yes. Hebrew can also say:

• אם נערום אותו גבוה מדי, ארגז הקרטון עלול ליפול.

That means the same thing.

The original sentence starts with the main point first:

• ארגז הקרטון עלול ליפול... = The cardboard box may fall...

Putting the condition first is also perfectly natural. It just changes the emphasis a little.