Learn Croatian: Činjenica da su bili zaboravili na sastanak jako nas zabrinjava. - The fact that they had forgotten about the meeting worries us a lot.

The subject of the verb zabrinjava is not 'they', but rather the singular noun činjenica (the fact). The entire clause Činjenica da su bili zaboravili na sastanak acts as a single, grammatically singular subject meaning 'the fact that they had forgotten'.

Could I use To što instead of Činjenica da at the beginning of the sentence?

Yes! You could say To što su bili zaboravili... and the meaning would be exactly the same. Both are used to create subject clauses, but Činjenica da sounds slightly more formal or emphatic than To što.

What tense is su bili zaboravili, and why is it used instead of the standard past tense (su zaboravili)?

This is the Plusquamperfekt (past perfect) tense. It is formed with the perfect tense of biti (su bili) plus the active past participle (zaboravili). It is used to emphasize that an action happened further back in the past, before another past point. You will see this often in writing at the B2 level, though in casual speech, Croatians often just use the regular past tense (su zaboravili).

Why do we say zaboravili na sastanak instead of just using the direct object zaboravili sastanak?