Can the structure of the u. s. government prevent a forming of a dictatorship?

Yes

Unless of all the structure is changed, the US Government has a hard time of changing because of certain reasons:

1) Checks & Balances: This is one of the key to stopping the formation of dictatorships, as each branch of the government has approximately the same amount of power, and can negate the effects of each.

For example, while Presidents can pass Executive Orders, the Judicial Branch can deem them unconstitutional, and void the orders. Presidents, if not using Executive Orders, have to pass their to-be laws through Congress, which can deem it unworthy, and vote it down. President's veto can also be overridden by a second 2/3 vote from each chamber (Senate and Congress).

2. Recall: Congress members are also affected, as they can be 'recalled', which is a vote that goes through the state, in which, if it garners enough votes, the member is replaced. If the judge in the judicial court does not follow rules, he/she can also be replaced by the president.

hope this helps

monarchy doesn’t have to have the hereditary element. there have been elective monarchies in the past where you choose your king-for-life and when he dies elect someone else for the job.