Questions & Answers about Det var det tuffaste loppet i världen.
Why does tuffast (toughest) have an -e at the end to become tuffaste?
Because it is describing a specific, definite noun ('the toughest race'). For regular superlative adjectives ending in -ast, you add an -e to make them definite. (Irregular superlatives, like bäst, take an -a instead). You also need the free definite article det before it.
Why do we have both the word det before the adjective and the -et ending on loppet?
This is the Swedish 'double definite' rule. When an adjective describes a definite noun, you must use a free-standing article (det because lopp is an ett-word) AND keep the definite suffix on the noun itself (loppet).
Both the sentence's subject and the article before the adjective are det. Do they do the same job?
No, they just look the same! The first Det is a pronoun meaning 'It' (as in 'It was'). The second det is the free definite article meaning 'the', which is required here to match the ett-word loppet.
Why is it i världen instead of just i värld?