I can't remember how many times I've told my students that stress - the emphasis placed on the sound or syllable spoken most forcefully in a word or phrase - is a very important aspect of spoken English. I tell them that although this linguistic feature may seem trivial to native speakers of Czech, it can be a matter of communicative survival in English. The trouble is that Czech has a fixed stress, meaning that its position can be predicted by a simple rule, i.e. it almost always comes on the first syllable. It's not a big issue if you place the stress elsewhere - you will likely be understood, provided you get other aspects of pronunciation right.
My students often struggle with sentence stress - the stress placed on words within sentences - and I wrote about ways of handling it here. They also find it difficult to deal with lexical stress - the stress placed on syllables within words. There are two notorious words I repeatedly correct - hotel and event. It doesn't matter how many times I model the pronunciation; in most cases my students will get it wrong the next time again. There are obvious reasons for this: as already mentioned above, it's natural for my students to speak stressing the first syllables in words. Moreover, despite the fact that in written Czech the word for hotel is identical to its English counterpart, we pronounce it slightly differently, i.e. we place the stress on the first syllable.
Now, my students are not the only ones who sometimes struggle with this aspect of spoken English. I remember at least two occasions when my message seemed totally unintelligible to my Australian friend, just because I placed the word stress incorrectly. For example, I remember that my friend looked really puzzled when I told him about the problem with mosquitoes. I pronounced it [ˈmɒskitəʊs] instead of [məˈskiːtəʊs]. I had to repeat the word several times and even describe the insect before my friend got the meaning. I was pretty frustrated because to my Czech ear, the difference is not so dramatic, and if I heard the word pronounced in different ways, I think I would always understand. By the way, this is one of the dangers of monolingual classes taught by a teacher speaking the same L1 - we understand one another and easily ignore things that seem unimportant to us.
Another communication breakdown happened when I used the word teetotaller. I said [ˈtiːtəʊtlə] instead of [tiːˈtəʊtlə]. Neither repeating the word nor raising a glass of beer helped my friend to get the meaning. I had to spell the word (which got me into even more trouble, as you can imagine)! I know that this isn't a very frequent word but this situation clearly demonstrates what an important role word stress can play.
I'm really happy I experienced those two communicative failures since I can share these stories with my students; I can show them what pitfalls there are waiting outside the safe L2 classroom.