Looks like a buttercup to me:-)

Thanks bornagain, is there a way to kill this as it came. Ack after I used vinegar.

There are weedkillers on the market that are designed to deal with lawn weeds without harming the grass, and these would be better than vinegar for you. The weeds look like creeping buttercup which is hard to kill even with that. Unfortunately you are supposed to use it only once a year so make sure you apply it properly as if you put on slightly less than the instructions say it won't be completely effective. At least that's what I've found.
Alternatively you could get some autumn Weed and Feed - this should help to thicken up the grass as well as discourage the buttercups, but it needs doing soon before the weather gets too cold for the grass to grow strongly.

Hi Aimankay,
I inherited a garden with it a few years ago, it is creeping buttercup and is usually symptomatic of heavy soil and water logged conditions, it thrives around me here. I have had good success, the plan is to spray the lawn in the spring with a broad leaf weed killer and keep on top of the mowing for the year, you can give it a second spray in July if you wish and thereafter once every two years or so. That will keep it in check but because it's indicative of the soil conditions it will return when you sit on your laurels. Hope that helps.

Thanks Steragram and Taurman.

I am aware that my soil is compacted and not drained. Since I was away, it wasn't aerated nor given any fees either. I had put vinegar first which killed most of the weed but probably not all of it. I have just put some ,20 kg to be precise,horticultural sands after aerating and taking the whole lawn. Then spreaded compost on top of grass seeds especially on those areas with burnt grass due to vinegar. Watered them and hopeful it will improve the soil condition. I also mixed some fertiliser and seaweed in the compost so that it gets some. Utriwnts as well.

Therefore, when I still saw buttercup regrowing I was surprised. Maybe it's early days and I need to dig the roots completely but will update. If you know of any better weed killer then do let me know. I had found that vinegar worked best for me for most other weeds when mixed with a little squirt of washing liquid.

Try Weedol for lawns. You won't get the black burned patches with that.