Learn Hardcore Portuguese (Brazil): É estranho que o elevador esteja tão silencioso hoje. - It is strange that the elevator is so quiet today.

Because é estranho que is an impersonal expression of opinion that triggers the use of the present subjunctive in Portuguese. The indicative está would not fit after this structure.

What is the present subjunctive and how do we form it for estar?

To form the present subjunctive of estar, start with the first-person singular present indicative estou, drop the final -o, leaving the stem est-, then add the subjunctive endings: -e, -es, -e, -emos, -eis, -em. That yields:
que eu esteja, que tu estejas, que ele esteja, que nós estejamos, que vós estejeis, que eles estejam.

Why do we use é estranho que here? Could we substitute another expression?

The phrase é estranho que expresses that the speaker finds the situation unusual or unexpected. It’s one of several impersonal expressions (for example, é bom que, é importante que, é provável que) that introduce a subordinate clause requiring the subjunctive. You could use é esquisito que or é curioso que, but you must keep the subjunctive mood.

Why is there no comma before que in this sentence?

In Portuguese, subordinate clauses introduced by que after an impersonal expression are not separated by commas. The combination é estranho que functions as a single unit linking to the subordinate clause.

Why is tão used before silencioso? Can we use muito silencioso instead?

Tão corresponds to English “so” and often emphasizes an unexpected or extreme degree. You could say muito silencioso (“very quiet”), but tão silencioso conveys surprise: it’s “so quiet (that it’s strange).” If you said o elevador é muito silencioso, it’s a neutral statement; tão silencioso adds emphasis and the idea of “strangeness.”

Why is hoje placed at the end of the sentence? Can it go elsewhere?

Adverbs of time like hoje are flexible in Portuguese and can appear at the beginning or end. Placing hoje at the end highlights the time context. You could say Hoje é estranho que o elevador esteja tão silencioso, but that shifts the focus and sounds slightly less natural than the original word order.

Why doesn’t estranho agree with elevador in gender and number?

Here, é estranho is an impersonal é + adjective structure introducing a subordinate clause. Because it describes the situation rather than directly modifying elevador, the adjective remains in the masculine singular.

Is there a way to express this with the indicative mood instead of the subjunctive?

Yes, but it changes the nuance. You could say:
É estranho, o elevador está tão silencioso hoje.
With a comma and the indicative está, it becomes a straightforward observation (“It’s strange, the elevator is so quiet today”) rather than the subjective reaction implied by the subjunctive structure.