Zdravím.

čtu o generálovi Stauffenbergovi a jeho nevydařeném atentátu.

He planned to take a briefcase bomb to one of these conferences . Once dictator was dead the troops in Berlin would seize key buildings and a new government would be declared .

Mohlo by být také were ? Plán mu nevyšel

Děkuji

Teoreticky by (snad, v krajním případě) mohlo, ale nemělo, čili it (possibly) could, but it shouldn't. Jestli se nepletu, jde tady o posun časů v nepřímé řeči, nikoliv o konjunktiv pro vyjádření hypotézy či domněnky.

• Once the dictator is dead, the troops will seize key buildings … → (His intention/plan/aim was that) once the dictator was /( were ) dead, the troops would seize …

“Were” tady po “once” ve smyslu “až bude …”, “jakmile bude …” zní dnes literárně, formálně až preciozně (nebo jako interference z jiného jazyka). Není (snad) nepřijatelné, ale je zbytečné, a skoro nikdo to nepoužívá, viz Ngram.

The English, bizarrely, use the past to express the future when the main clause is in the conditional (zdroj):

• He will find out when [/once] he is twenty-five.
• He would find out when [/once] he was twenty-five.

Další příklady:

Caesar was once kidnapped by pirates while still a young man. He joked with them that he would have them executed once he was free.

Although Grant occasionally worked in the tannery as a child, he hated the work and swore to his father that once he was an adult, he would never do it again.

But he would also always promise, every time we met, that once he was out of office he would write a play about the comedy of high politics …