Buttercups

The first three above usually start flowering at the beginning of May, while goldilocks starts about a month earlier and is usually found in woodland where it needs to flower before the trees start to shade it as their leaves break. The characters to look for with goldilocks are an erect stem bearing simple three lobed leaves, almost kidney shaped leaves at the base but dissected ones up the stem. The flowers are bright yellow as in all the buttercups, but often, though not always, appear as if one or two are missing from the five you would normally expect. Finally, look at the seeds because they are hooked a bit.
The corn buttercup , being an annual species growing with cereals, flowers when the corn is ripening, has small flowers and then large seeds.
Now the other three. The common buttercup (Ranunculus acris) is rather unpalatable to cattle, so it is often left uneaten, they don't like the taste. The stems are erect, up to 80cm, often with a whitish bloom and branched to produce an inflorescence of bright yellow flowers, and the leaves are delicately dissected. Look for two features. The sepals are held up against the petals like a greenish yellow cup and the flower stalk is round and smooth.
The bulbous buttercup(Ranunculus bulbosus), like the common buttercup grows mainly in grassland, and flowers in the first week in May making fields bright yellow instead of green because they occur in such huge numbers. It is called bulbous because the stem is swollen just below ground level, so you can't see that without pulling the whole plant up. Look for the sepals, which quickly become reflexed (folded back on themselves) and the flower stalk which is ridged rather than smooth as in R. acris.
Thirdly, the creeping buttercup (Ranunculus repens). As its name suggests it tends to be a prostrate species, with stems on the ground rooting at the nodes , rather than erect as in the two previous species. This varies, depending upon the nutrient status where it is growing. The flower stalk is ridged as in R. bulbosus, but the sepals are not reflexed. It sometimes has more than the five petals one would expect and the extra ones are often smaller than the others. Look for trailing stoloniferous stems rooting at the nodes and variously dissected leaves which have colour variegation at the base of the dissections of the leaf.
A fifth species which is now very rare. It is the Corn Buttercup – (Ranunculus arvensis). It is an annual which used to grow in cereal crops, and is rare because farmers do not allow their cereals to become weedy. It is a slender plant which can grow up to 60cm if the nutrient status is high and there are tall plants, like cereals, to compete with. It has small yellow flowers and is notable for its spiny seeds . The one pictured has a strange history. When the A127 road was being widened near Basildon in Essex, one plant appeared on the road verge where the soil had been disturbed. Seed was collected from it, multiplied and distributed. I have it growing in a plant pot in the garden and expect it to flower at the end of May.