Ovo prvo - Jutra su teška.
If you need a bit more explanation - 'teška' goes in same case as 'jutra', nominative plural for noun of neutrum gender ending with o (jutro > jutra) so 'teško jutro' (nom sing) > 'teška jutra' (nom plural)
More or less :)
2017년 11월 3일
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Jutra su teška.
Because teška is an adjective, and you need an adjective as you describe a noun.
Teško is an adverb (hardly, heavy). E.g. Teško se krećem. (I can hardly move; I move hardly). An adverb describes a verb.