Do I have to submit Form 720 Spain again if I sell securities that I had already declared in a previous Form 720 Spain?
Yes. You must submit Form 720 Spain to indicate that you no longer own the securities you have sold.
Should I submit Form 720 Spain if I am the heir to a recumbent inheritance?
Yes, and you should declare the recumbent inheritance given that it is considered a company.
If in 2018 I have the following assets: a) An apartment abroad that is sold halfway through the year for 100,000 euros; b) A bank account abroad with an average balance in the last quarter of 60,000 euros and a year-end balance of 42,000 euros; c) An investment fund valued at 55,000 euros. What assets should I declare in 2019?
You have to declare the investment fund and the bank account.