Water temperature and why it matters
Green tea is the category most easily ruined by water that is too hot.
The leaf is unoxidised, so nothing has softened it before it reaches the cup.
Boiling water pulls catechins and caffeine out fast, and bitterness arrives with them.
A cup brewed at 100 °C tastes flat and harsh within 90 seconds.
The same leaf at 80 °C tastes sweet and grassy instead.
That single difference is why many people think they dislike green tea.
Most Chinese green teas want water between 80 and 85 °C.
Japanese greens, sencha especially, want it cooler at 70 to 75 °C.
Gyokuro is brewed cooler still, and it is thick rather than brisk when it is right.
Matcha is whisked rather than steeped, and it uses warmer water than sencha does.
If you have no thermometer, boil the kettle and let it stand.
Several minutes off the boil lands close to where you want it in a cold room.
A thermometer costs very little and settles the question permanently.
Temperature is the one variable worth getting right before anything else.
Leaf, water and time
The standard starting point is 3 g of leaf in 240 ml of water.
That is roughly a level teaspoon in a mug, and it is easy to remember.
Chinese guides often use less water for the same leaf, which gives a stronger cup.
Either approach works, and the difference is concentration rather than correctness.
Steep for 2 minutes at 80 °C, then taste it before deciding anything.
Most green tea is ready somewhere between one minute and three.
Past that point astringency builds faster than sweetness does.
Green leaf also gives several infusions, which most black tea does not.
The second steep often tastes better than the first, since the leaf has opened.
Later steeps can run a little longer to keep the strength up.
Three infusions is a reasonable expectation from a decent leaf.
After that the flavour thins, and the leaf has given what it had.
Weigh the leaf once so you know what your usual dose looks like.
Time is easier to adjust than temperature once you have a starting point.
Adapting to the style
Leaf shape tells you most of what you need to know.
Flat, pan-fired leaves such as Longjing（龙井）are dense and release slowly.
They tolerate slightly hotter water and a longer first steep.
Needle-shaped and downy leaves release fast, and they want cooler water.
Biluochun（碧螺春）is small and tightly rolled, and it is ready quickly.
Rolled Japanese leaves sit somewhere between those two.
Bud-heavy picks are delicate, and they scorch more easily than open leaf does.
Broken leaf, which is what a bag usually holds, brews fast and turns bitter faster.
That is why bagged green tea rarely tastes like the loose version.
Roasted greens behave differently again and take hotter water happily.
Powdered matcha needs no steeping at all, and no patience either.
When in doubt, start cooler and shorter, and adjust upward from there.
A weak cup can be corrected, and a bitter one cannot.
Shape matters more than origin when you are deciding how to brew.
Fixing a cup that has gone wrong
Bitterness is the usual complaint, and it has three causes.
The water ran too hot, the steep ran too long, or there was too much leaf.
Fix the first by letting the kettle stand before you pour it.
Fix the second with a timer, and a phone does that job well enough.
Fix the third by weighing the leaf rather than guessing at it.
A small scale is cheaper than a thermometer and more useful than most teaware.
If the cup is already bitter, adding water helps less than people hope.
It dilutes the flavour along with the bitterness.
A pinch of salt is an old trick, and it masks the problem rather than removing it.
Cold brewing avoids the issue entirely, since cold water pulls far less catechin.
Use 5 g in a litre, leave it overnight, and strain it in the morning.
The result is sweet and mild, and it is hard to get wrong.
A weak cup is the easy problem, and bitterness is the one worth avoiding.
Most problems with green tea come down to heat rather than to the leaf.
Get the water right first, and the rest tends to follow.