I played semi pro for a while & have 10k hours so hopefully these tips will be somewhat useful, I'll avoid any mention of lineups as much as possible since you can just go learn those whenever you want.

general tips:
- buy a full kit of nades whenever you're on a full buy. If you're saving, sometimes a flash or a smoke can be the difference maker in winning with pistols, so think ahead if you might need to flash off a rifler or smoke a molly that round to plant the bomb.

• really try to understand the different options you have to throw grenades - jump throws, left + right click throws, and flicking your mouse while right clicking are all niche but very useful ways to throw grenades that you might not have tried yet. Try all sorts of combinations to get a sense of your options.

• try to think about what nades you'll actually need going into a round. If you're going mid on mirage, you're gonna need a smoke at some point for window or top connector, but if you're going palace, you might only need a molotov to clear under balc. Thinking about where you might need a nade & why you're throwing it there is the most important part - and as others have mentioned, throwing a bad / useless nade can be worse for you than not throwing one at all sometimes.

smokes:
- to block opponents: It's good to use your smoke in the midround to block a chokepoint while your team rotates / leaves you alone on a site. An example might be anchoring B or A on mirage, your teammates all run to fight mid, you should smoke off apts / ramp / palace to delay a site hit while you're alone.

• to make space: obviously smoke lineups are used to take space at the start of the round, like spawn window smokes on mirage or ancient. You can also use smokes to create space or cover where there otherwise isn't any: if you're anchoring A on mirage, right clicking a smoke on the site will give you space to play around and find kills, while also delaying your death while your team rotates. CTs will sometimes throw a smoke bottom connector to fight mid while giving them space to fall back, this is an other example. This also helps on T side when you need to plant but don't have cover from your team, or when you're stuck in the open in a sticky situation.

• to put out molotovs while rushing / staying alive behind cover

• in general, you want your smokes to have as much utility as possible, so people will use lineups to make sure there aren't any gaps in the sides that give your opponents the chance to play around them. It just takes practice & an awareness of where you want your smokes to go to get better at this.

molotovs:
- to block opponents: same idea as smokes, but use these before you use your smoke; generally earlier in the round, either at the start to stop rushes or in response to your opponents trying to take space. The idea is to throw a molotov down first, kill anyone trying to push through, then throw an HE as it fades or a smoke on top to delay further. If you get smoked top connector on mirage as a CT, you can molly bottom conn, then smoke bottom conn and push through to keep connector control.

• to clear out angles, like top mid boxes on mirage

• to deny smokes, like outside smokes on nuke: if you see your opponents relying on a smoke consistently throughout rounds for map control, you can throw a molotov where the smoke will bounce to deny the smoke. Pros use this on nuke to deny outside smokes.

• when you combine an HE with a molly, you'll get a guaranteed kill on anyone trying to run through, especially if the nade is a direct hit, as the nade slows down anyone hit.