Tantum abest ut fornax frigida sit, ut aeneum iam calefaciat.

Because tantum abest ut ..., ut ... is a standard Latin construction.

• The first ut introduces the idea being rejected: ut fornax frigida sit.
• The second ut introduces what is actually true instead: ut aeneum iam calefaciat.

So the sentence is built as a contrast: so far is it from being the case that X, that instead Y.

Literally, abest means is away, is absent, or is far from, and tantum means so much.

Together, tantum abest ut ... means something like it is so far from being the case that .... In smoother English, this often becomes far from ... or so far from ... that ....

You can think of the whole opening as an idiom rather than translating each word too mechanically.

Formally, tantum is the neuter accusative singular of tantus. Here it is used adverbially, with the sense by so much or to such a degree.

So tantum abest is literally something like it is distant by so much. In practice, though, learners usually do best to recognize the whole phrase tantum abest as a set expression.

Both verbs are in ut-clauses, and in this construction Latin normally uses the subjunctive.

• sit is in the first clause, the idea being denied.
• calefaciat is in the second clause, the contrasting reality.

So the subjunctive here does not mean the action is doubtful in the English sense. It is mainly a matter of Latin syntax: this idiom calls for the subjunctive.

Because frigida agrees with fornax.

• fornax is feminine singular.
• Therefore the predicate adjective must also be feminine singular: frigida.

It is nominative because it goes with the subject after sit: the furnace is cold.

The subject is understood from the context: fornax.

Latin often leaves out a repeated subject when it is obvious. So in ut aeneum iam calefaciat, the sense is that it, meaning the furnace, is already heating the bronze vessel.

Aeneum is the direct object of calefaciat, so it is accusative singular.

Here it is being used as a noun, meaning something like bronze vessel, bronze pot, or kettle. If you know aeneus, -a, -um as an adjective meaning bronze or made of bronze, this is a normal Latin habit: an adjective can stand by itself as a noun when the noun is understood.

Iam means already or by now.

It strengthens the contrast. The point is not just that the furnace is not cold; it is so much the opposite that it is already heating something else. So iam adds a sense of immediacy and emphasis.

A good way is to bracket the sentence like this:

Tantum abest
ut fornax frigida sit,
ut aeneum iam calefaciat.

Then read it as:

• So far is it from the case that the furnace is cold
• that it is already heating the bronze vessel

Latin puts tantum abest first to signal the idiom right away. Once you recognize that pattern, the rest of the sentence becomes much easier to follow.