Sedges have edges, rushes are round
And grasses are hollow right down to the ground
This mnemonic for distinguishing between sedges, rushes and grasses is worth remembering, even though there are plenty of exceptions to the rule.
Basically, it is the many Carex spp sedges that have the edges i.e. their stems are more or less triangular in section, a characteristic that is usually easy to see and certainly easy to feel with your fingertips. The French for sedge is laiche.
To identify them reliably to species level it is best to run the seed capsules through a key. You will need to learn some technical language relating to the parts of the plant and a 10 or 20x magnifying loup to see the level of detail required. Each species has a distinctive grainlike seed (technically a 'fruit'). Useful references books are the Colour Identification Guide to the Grasses, Sedges, Rushes and Ferns of the British Isles and north-western Europe by Francis Rose, the Field Flora of The British Isles by Clive Stace and the BSBI Handbook Sedges of the British Isles. They are not easy to identify from photographs.
Glaucous Sedge C. flacca: La Laiche glauque in French. Very common, with flattish leaves that are generally greyer or duller on top than underneath. Found in calcareous grasslands, meadows, fens, flushes. Fruits June-September. Below, Glaucous Sedge photographed flowering in the Parc de Boussay, April.
Hairy Sedge C. hirta: la laiche hérissée (='the bristly sedge'). Hairy (although less so if the site is wet). The fruits are long beaked and downy, the leaves grow in tufts and can be flat or keeled (v shaped in section). The female spikelets are widely spaced and each has a long leaf like bract. The plant grows in meadows, hedgebanks and open woodland. Fruits June - September. Common. The photos below are of a plant in the draw down zone of the Etang de Ribaloche in the Foret de Preuilly, June.
Cyperus Sedge C. pseudocyperus: known in French as la laiche faux souchet (='the false papyrus sedge'). Loosely tufted, growing 40 - 90 cm tall, with rough edged sharply angled stems. The leaves are bright shiny yellowy green, rough to the touch, very long (over a metre) but fairly rigid and upright. The female spikelets are cylindrical, drooping, bright green and prickly looking with long leaf like bracts. The species is fairly common in swamps, ponds, ditches and woodland pools, avoiding acidic waters. The photo below was taken in June. The plant is growing in the draw down zone of the Etang de la Rolle in the Foret de Preuilly.