You actually can carry as support
Even if your ADC is unskilled, you can often win lane as support, especially if the enemy ADC is similarly skilled. It doesn't mean you have to get kills. If you can manage to get your ADC to a significant experience advantage and/or gold advantage (from denying CS to the enemy), your ADC will naturally win the lane for you simply by being stronger when trades happen.
I am not a highly aggressive player, but as support I always try to be in a position to make plays. If the enemy ADC steps out of line, I can potentially capitalize for very big damage or at least chip away at some health with my autoattack or a quick spell.
However, not every Grasping Roots I land is something I want to go in on. It's the same with Light Binding, Dark Binding, or even Thresh and Blitzcrank hooks. Sometimes your ADC is not in the right place to confirm, and sometimes the enemy ADC is in a good spot to retaliate. Sometimes you land a Thresh hook on a tanky support like Leona, and if both ADCs go in, Thresh will be the first to die.
This is extremely important for a character like Thresh who can decide how hard he wants to go in on his combo starter, but it's also important for the ADC to understand. Sometimes for ADC it's best to just wait for the enemy ADC to react and aggress on her so your support can get away. Hopefully your support understands that she needs to get away.
This is why I prefer aggressive supports but not those who require a huge commitment to start an exchange. I like Zyra, Lulu, or Janna a lot more than Leona or Blitzcrank. For Leona, it is hard to communicate when your Zenith Blade is safe harassment (hitting the enemy support when neither ADC can engage immediately) and when you're looking to confirm a kill.
Read your partner's intentions
Unless you're in voice chat, it's really important to understand the intentions of your lane partner. In general, it's better to play in a relaxed, passive-aggressive style if you are solo queueing. There is no guarantee that your partner will respond to your aggression, which is again why I prefer Zyra to Blitzcrank.
If I land a Grasping Roots combo but my ADC is asleep, I still deal a ton of damage, and I'm probably completely safe. On the other hand, if I land a Rocket Grab as Blitz and the enemy goes all-in on me, I will probably lose much more health unless my ADC is awake and in a good spot to confirm. It's a lot easier to confirm a combo off a Blitz grab than it is a Zyra snare, but this leads to more options for the ADC as well. If the snare/combo goes down on the enemy support, Zyra's ADC can wait for the enemy ADC to chase Zyra or shoot the plants and get some free damage in.
If I'm playing ADC and my support is very mellow, I will usually exert just enough lane presence to encourage the enemy to not go all-in on me. I will happily relax and farm while waiting for jungle presence, and if my support ever does something wild like a Sona Flash-Crescendo from out of nowhere, I do my best to work with it.
If my support is very aggressive, it gets much harder. It's often difficult to identify the degree to which I should engage on anything my support does. Aggressive supporting is nice, but too often those players try to make plays that aren't there. It's so important to understand where all players are before a support tries to make that winning play.
If I'm supporting and my ADC is very aggressive, I try to be more of a bully as a support. This doesn't necessarily mean that I turn on the kill switch too, but I definitely try harder to keep the enemy players from laning. Sometimes it goes fabulously and you zone both enemy players out of EXP range as Sona/Caitlyn, but other times you're a double kill waiting to happen.
If my ADC is very passive, I still try to put on pressure, though. If I am not sure that my ADC will land plays, I spend extra effort watching the minimap rather than trying to start something that won't finish. While a support should always watch the minimap and announce MIAs and give danger pings, it makes sense to spend more time keeping an eye on it if there's nothing to do in lane.
Understand role strengths and weaknesses
ADCs are very weak early game. Their puny autoattacks deal mediocre damage at level 3 compared to a rank 2 spell from a support. Even ADC spells tend to have low base damage and higher ratios. Take Lulu for instance; a Help, Pix! into Glitterlance combo deals around 200 magic damage and happens virtually instantaneously, and that doesn't include her stronger-than-usual autoattack that will probably chunk off another 60+ HP. Zyra and Lux combos are even more damaging.
This means that until the ADC is carrying a BF Sword, the support is probably going to be running the show. And for a support, it's important to know exactly where your damage is and how big it is. For instance, the Lulu combo above (Help, Pix! plus Glitterlance plus a basic attack) deals roughly a third of the enemy's life at level 4. At level 6, Lux can deal a pretty monstrous amount of damage (roughly 700) with Q > auto > E > burst > R. If she gets to use her passive more than once, she deals at least 100 extra damage for every hit. That's enough to -- without any AP scaling at all -- one-shot a character before magic resist is considered.
However, once items start coming into play, the ADC plays a big role. With just shoot > tumble > shoot > BoRK > shoot, Vayne can deal extremely heavy damage once she finishes Blade of the Ruined King, even without any other items. If she uses Condemn or her ultimate, that damage increases even more.
What this all means is that supports (excepting Lulu and Sona, to a degree) are very reliant on up-front burst. If they can't kill the enemy with their initial spells, it's probably wrong to expect more from their 50-60 damage autoattacks. Likewise, ADCs can't take as much advantage of little harassment windows as a support can. Sona can chunk off 200 HP at level 3 if the enemy steps into her Q range, more if she can do a Power Chord attack reset. In the meantime, Caitlyn is shooting her little peashooter for under 70 damage.
There's so many elements to working with your partner that we could probably create an entire new column just to talk about lane teamwork. It's vitally important to do your best to work with your partner and try not to make things hard on him or her.
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