Learn French: Si glissant que soit le glacier, ils vont traverser la crevasse. - However slippery the glacier may be, they are going to cross the crevasse.

It does! But in this specific structure—si followed by an adjective and que—it expresses concession. It translates to 'however [adjective]' or 'no matter how [adjective]'. So si glissant que means 'however slippery'.

Why do we use the subjunctive soit here instead of the indicative est?

The structure si + adjective + que (meaning 'however') always triggers the subjunctive mood. It frames the slipperiness as a condition being conceded rather than a simple statement of fact, so the verb être must be conjugated as soit.

Why is the verb placed before the subject (soit le glacier) instead of the normal order (le glacier soit)?

In this specific structure, it is highly idiomatic to invert the verb and the noun subject. You can technically say si glissant que le glacier soit, but si glissant que soit le glacier flows much better and is the heavily preferred word order in written or polished French.

Can I use aussi instead of si in this sentence?

Yes, absolutely! Aussi glissant que soit le glacier means the exact same thing. In fact, using aussi is more common in everyday spoken French, whereas si adds a slightly more formal or literary tone to the sentence.