Learn Croatian: Osjećam veliku radost jer si ovdje. - I feel great joy because you are here.

Normally, feminine nouns take an -u ending in the accusative case. However, radost belongs to a special group of feminine nouns that end in a consonant (the i-declension). For these nouns, the accusative singular looks exactly the same as the nominative singular. That is why it stays radost instead of becoming radostu.

Why does the adjective take the accusative form veliku if the noun radost doesn't seem to change?

Adjectives must always match the noun's gender, number, and case. Even though radost doesn't physically change its spelling in this sentence, it is still grammatically in the feminine accusative case. Therefore, the adjective velika changes to its regular feminine accusative form, veliku. Seeing a mismatch in endings like veliku radost is a very common hallmark of these consonant-ending feminine nouns!

Why is the short verb si placed right after jer?

Jer (because) is a conjunction that introduces a brand-new clause. Enclitics like si strictly require the second position in their clause. Since jer occupies the first spot of this new clause, si is pulled directly into the second position.

Could I use tu instead of ovdje for 'here'?