Tantum abest ut suspensura cadat, ut nunc firmior sit.

It is a fairly common Latin idiom meaning:

• so far is it from being the case that X, that Y
• more naturally in English: far from X, Y instead

So here the sentence does not simply say one thing and then add another. It sets up a strong contrast:

• far from the suspensura falling
• it is actually firmer now

This construction is often stronger and more emphatic than a simple negation.

Because they do two different jobs.

1. The first ut clause gives the thing that is being rejected:

   • ut suspensura cadat
   • that the suspensura should fall / is falling

2. The second ut clause gives the actual outcome or opposite reality:

   • ut nunc firmior sit
   • that now it is firmer

So the pattern is:

• tantum abest ut X
• ut Y

Literally: it is so far from X, that Y

In English, we often smooth this out as far from X, Y instead.

Because this whole expression regularly uses the subjunctive.

• cadat is subjunctive in the first ut clause because it depends on the idiom tantum abest ut
• sit is subjunctive in the second ut clause because that clause is a result/consecutive clause

So this is not a purpose construction. Even though ut very often introduces purpose, here that is not what is happening.

A good rule for a learner is:

• after tantum abest ut, expect a subjunctive
• in the matching second ut clause, also expect a subjunctive

No.

That is a very natural question, because learners often first meet ut + subjunctive as purpose. But here the first clause is not purpose.

ut suspensura cadat does not mean in order that the suspensura fall.

Instead, it means something like:

• that the suspensura should fall
• for the suspensura to fall
• the suspensura’s falling

It is the content of what is being denied by tantum abest.

Here abest is used in an almost impersonal way.

Literally, abest means is away, is absent, or is far from. In this idiom, Latin uses it like this:

• tantum abest ut ...
• it is so far from being the case that ...

So in English we naturally supply it as the subject:

• it is far from the case that ...

You do not need to look for a normal concrete noun subject for abest.

Tantum is the neuter accusative singular of tantus, but here it is being used adverbially.

So it means:

• so much
• by so much
• so far

In other words, it is not a direct object. It functions more like an adverb modifying abest.

So:

• tantum abest
• it is so far from it
• it is very far from being the case

Here it is best taken as a noun.

In technical or architectural Latin, suspensura can mean something like a suspended structure, especially a suspended floor or similar construction.

In this sentence it is:

• nominative singular
• the subject of cadat
• also the understood subject of sit

A learner might wonder whether it could be a participle because of the ending -ura. In isolation, that form could look participial, but in this context the noun meaning makes much better sense.

Because firmior is an adjective agreeing with suspensura, which is feminine singular.

• suspensura = feminine singular
• therefore firmior = feminine singular comparative adjective

So firmior sit means:

• may be firmer
• is firmer

If it were firmius, that would be neuter, and it would not agree with suspensura.

Because Latin often leaves out a repeated subject when it is obvious from context.

The subject of sit is still suspensura, understood from the first clause.

So the sentence is effectively:

• ut suspensura cadat
• ut suspensura nunc firmior sit

But Latin does not need to repeat suspensura, because the reference is perfectly clear.

Because the negative idea is already built into the expression tantum abest ut.

Latin does not say:

• tantum abest ut non cadat

to mean far from falling in this sentence.

Instead, the idiom itself already gives the force:

• tantum abest ut cadat
• it is far from the case that it falls

So the absence of non is normal here.

Because the sentence is talking about a present situation.

• cadat = present subjunctive
• sit = present subjunctive

This gives the sense:

• far from falling
• it is now firmer

The word nunc especially points to the present: now.

If the sentence were framed in past time, Latin might use imperfect subjunctives instead. But as written, the present subjunctive fits the current state being described.