Short answer: an uptrend makes higher highs and higher lows; a downtrend makes lower highs and lower lows. The trend stays intact until price closes past the last swing that defines it — the last higher low in an uptrend, the last lower high in a downtrend.
A swing high is a peak with lower candles on both sides; a swing low is a trough with higher candles on both sides. Ignore the small wiggles — mark only the swings that stand out on your timeframe. Then read them in order: are the highs and lows stepping up, stepping down, or going sideways?
A break of structure is a close past the last swing in the direction of the trend — a new higher high in an uptrend, a new lower low in a downtrend. It says the trend is still being pushed. A break on a strong candle with volume behind it carries more weight than one that barely closes through.
When price closes past the swing that defined the trend in the other direction — under the last higher low in an uptrend — the structure has changed. It does not mean a new trend has started; it means the old one is no longer intact. The first retest of the broken swing often decides which way it goes.
After a straight move there may be no swing near price at all — a coin that dropped 60% without a bounce has its last lower high at the top of the move. Then the useful line is the nearest level: a close back over the first ceiling is the first sign sellers are losing their grip, long before the trend formally breaks.
Every Sylvo market read opens with this structure — for example the Bitcoin and Ethereum pages.
A swing low that sits above the previous swing low. Higher lows show buyers stepping in earlier each time, which is what keeps an uptrend intact.
A break of structure (BOS) continues the trend: a new high in an uptrend. A change of character (CHoCH) breaks the swing that held the trend: a close under the last higher low in an uptrend.
Your own timeframe for the details, and one timeframe up for the direction. A 4-hour uptrend inside a daily downtrend is usually a bounce.