Răspunderea companiei aeriene este mare.

Because răspunderea is the definite form of răspundere.

In Romanian, the definite article is usually attached to the end of the noun:

• răspundere = responsibility / liability
• răspunderea = the responsibility / the liability

So here the sentence is talking about a specific thing: the airline’s liability/responsibility.

It is in the genitive singular.

Romanian often uses the genitive where English uses of or ’s. So:

• răspunderea companiei aeriene
  literally = the liability of the airline
  naturally = the airline’s liability

The key form is companiei, which shows that companie is functioning as the possessor.

Because compania is the nominative/accusative definite form, meaning the company as a subject or direct object.

Here, the company is not the subject. It is the owner/possessor of the liability, so Romanian uses the genitive:

• compania aeriană = the airline
• companiei aeriene = of the airline / the airline’s

A useful contrast:

• Compania aeriană este mare. = The airline is big.
• Răspunderea companiei aeriene este mare. = The airline’s liability is great/high.

Because the adjective must agree with companiei in gender, number, and case.

The basic form is:

• companie aeriană = airline

But in the genitive singular, it becomes:

• companiei aeriene = of the airline

So aeriene is the feminine singular genitive/dative form that matches companiei.

Romanian usually expresses both of those ideas through word endings, not separate words.

• English: the airline’s liability or the liability of the airline
• Romanian: răspunderea companiei aeriene

Here:

• -a in răspunderea marks the
• -iei in companiei marks the genitive idea, similar to of the or ’s

So Romanian packs a lot of grammar into the endings.

Mare is simply the correct adjective form here.

Two important points:

• mare is the same for masculine and feminine singular
• after este, adjectives are normally used without the definite article

So:

• este mare = is big / great / high

Not:

• este mară — incorrect
• este marea — not correct in this structure

The definite article belongs on the noun when needed, not on the predicate adjective here.

Not literally in a physical sense.

With abstract nouns like răspundere, mare usually means something like:

• great
• high
• significant
• serious
• extensive

So răspunderea ... este mare means the liability/responsibility is substantial, not physically large.

In a normal statement like this, este is expected.

• Răspunderea companiei aeriene este mare. = fully standard
• Răspunderea companiei aeriene e mare. = also very common, with the shorter spoken form e

Unlike in some languages, you normally do not drop the verb to be in this kind of Romanian sentence.

Yes, but the given order is the most neutral:

• Răspunderea companiei aeriene este mare.

This is basically:

• subject: Răspunderea companiei aeriene
• verb: este
• predicate adjective: mare

You can change the order for emphasis, for example:

• Mare este răspunderea companiei aeriene.

But that sounds more emphatic, literary, or marked. The original version is the one learners should treat as the default.

Yes, that is possible, but răspundere and responsabilitate are not always identical.

Very roughly:

• răspundere often suggests liability or accountability, especially in legal or formal contexts
• responsabilitate often suggests responsibility in a broader, more general sense

So in a legal, corporate, or institutional context, răspunderea companiei aeriene can be especially natural.

The forms show it:

• răspunderea = singular definite
• companiei = singular genitive
• mare = singular adjective form here
• este = singular is

A plural version would look different, for example:

• Răspunderile companiilor aeriene sunt mari.

That would mean The airlines’ liabilities/responsibilities are great/high.