Foreigners, particularly well-educated ones, can use “big” words incorrectly. They can litter their sentences with words like RELEVANT or COHESION and misuse these words. In helping foreigners to write better, I usually try to get them to simplify their sentences.

Foreigners can also use idioms correctly but almost comically. They might throw in “kick the bucket” in a serious context. When they jam in too many idioms it can sound off to us natives. Time flies, a bird in the hand, that sort of thing.

Russians don’t use definite articles and will omit them. Indefinite too, if I’m not mistaken. I’ve caught Iranians–who speak Persian or Farsi–also doing this.

Also you might find verbs placed in unusual positions.

Another thing that tends to occur is that non native speakers may use the ‘ing’ ending in odd ways.

Maybe the order of words? Maybe sayings that don’t translate well?