"I see the sea."
Translation:Я вижу море.
i thought in russian you could say for example znayu or dumayu to mean i know or i think
Yes, we can say this way. But it would not be a translation of this sentence from the task, which has "я"
It is understood that the given translation has я in it, the question is why is the omission of я marked incorrect here?
Because the sentence from the task has "I" and you have to translate it to Russian. But try to report your choice.
"Смотреть" is "to watch," or "to look at" when used with the preposition "на."
Indeed this sentence does require the accusative case. Морю is actually the dative case of море. You can see all the declensions of this word here https://en.wiktionary.org/wiki/%D0%BC%D0%BE%D1%80%D0%B5#Russian
it is already accusative, but because море is neuter, it doesn't change: http://www.russianforfree.com/grammar-of-russian-nouns-accusative.php