I started investing in US stocks and lost 15% of my capital due to lack of close monitoring. Can anyone shares how should a passive beginner invest in US stocks?

If you are not monitoring actively, you are investing long term. Go for ETFs like VOO, SPY or QQQ. Or go for robo investors like Syfe or Endowus.

Depends on your strategy. If it is not speculative, then no need to keep monitoring.

Etf is a good place to start. Important is to zoom out of the timeframe and not be too fixated into the short term volatility. DCA consistently is the best way to beat the market

It depends on what you invest in; such as specific company or ETF, is the decline due to specific event or?

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You would need to know the issue to avoid it or manage such volatility :)