Learn Swedish: Skateboarden rullar bra, men själva asfalten är blöt. - The skateboard rolls well, but the asphalt itself is wet.

When you want to say 'the actual [noun]' or 'the [noun] itself' to contrast it with something else (here, the skateboard), you use the invariable form själva placed directly before the definite noun. If you used själv right after the noun (asfalten själv), it would sound like you are personifying the street, as if the asphalt did something 'personally' or 'by itself'.

How do we know to treat the English loanword skateboard as an en-word (skateboarden)?

Most English loanwords that refer to physical objects, tools, or vehicles are adopted as en-words in Swedish. Because it is an en-word, it takes the standard -en ending in the singular definite form to become skateboarden.

Would it be wrong to say asfalten är blött?

Yes, it must be blöt. Asfalt is an en-word (which you can tell from its definite ending in asfalten), so the adjective must agree with it by taking the base en-word form. You would only use the t-form blött if you were describing an ett-word, such as vädret (the weather).

Could I use våt instead of blöt in this sentence?