• A stain is a mark, caused by a liquid. If you spill coffee on your shirt, it will stain your shirt. • A smudge is a small mark, usually caused by by something more solid. If your shirt's sleeve brushes dirt while gardening, you might have a smudge on your sleeve. Likewise, if you touch your laptop screen with your finger, you might leave a smudge of finger oil on it. In addition, you can 'smudge' something liquid, such as if touch a painting before it dries, you may smudge the paint. • A fleck is something that is more mobile, such as a fleck of dust or dirt. If you lightly brush it off, it will go away. If you are removing dried paint from your walls, a fleck of paint might land on you, but you can brush it off your shirt lightly and it will not leave any stain. In your sentence "Construction workers left (Smudge, stain or fleck) of foam on my floor, I have to clean it up.", I am not sure what exactly the foam is like, but I believe in any case you can just say they "left some foam on my floor" and that should be fine. You don't need smudge, stain, or fleck in this case, unless the foam itself stained your floor. I'm a native English speaker and a community teacher on iTalki, so if you want any practice speaking English, feel free to check out my profile https://italki.com/adrian.linglish

Following on from Adrian's explanation, I'd say that 'fleck' is the only one of the three options that works here. 'Fleck' does collocate with 'foam', both as a noun and as a verb/verbal adjective: for example, it is common to talk about the sea being 'foam-flecked'. 'Smudge' would be appropriate for paint, mud, oil or a dirty, greasy or sticky substance - not foam. 'Stain' would be a mark that's either be permanent or troublesome to remove - definitely not for foam.

Stain

Hi Bogdan. It is understandable that these words have you scratching your head. :) So let's give this a try: Smudge - usually involves the smearing or rubbing of the undesirable item on a surface like if a woman leaves a little bit of her lipstick on your cheek when she presses a kiss there or maybe a fingerprint on the lens of your glasses. Fleck - this is a very small dot or spot and is sometimes intentional in fabrics where the base color is dark with regular or irregular tiny specks or flecks of a contrasting color. It is also a painting technique where the spots of paint on a surface are created by flicking the brush bristles to release a fine spray of paint droplets. Stain - the primary distinction with stain is its nearly permanent condition. This may be a discoloration in your shirt where you spilled red wine. If not treated immediately, it may never be released from the fabric. And don't forget in construction the use of products called 'stains' to permanently color woods...hence the name. :) Hope this helps, e.