What makes a non-native English speaker sound foreign? [closed]
Solution 1:
Obviously without hearing an example, it's only possible to talk in generalities. In terms of syntax and vocabulary, even if you don't produce any utterances that are ungrammatical per se, you might give yourself away by not using features (e.g. filler words/pharses like "you know", "though", "actually") that are common in native speech, or by using features not common to the spoken register (e.g. overuse of non-contracted forms).
In terms of pronunciation, native speakers are tuned to all sorts of fine phonetic detail. So even if, say, the quality of your vowels is very close to that of a native speaker, you could give yourself away by other subtle clues: e.g. subtle differences in the duration of sounds, having different patters of Voice Onset Time to native speakers, not glottalising vowels before a syllable-final voiceless consonant, not overlapping segments of consonant clusters such as [kt] in the same way as native speakers of English, to mention some common phenomena that tend to differ between English and other languages but not so much within accents of English and which can give an accent away as "foreign". But really there are virtually too many possible subtle differences to mention, and surely some that haven't been systematically studied.
Solution 2:
Based on your written English you are doing very well indeed. Our ears and brains are very attuned to tiny details in language, because we use it all the time. So, although I know it is not the answer you want, it is a combination of hundreds of little things that make a non native speaker sound non native. It should also be said that this is true of accents in general: what makes an Irishman sound different than an Englishman? What makes a Western Australian sound different than an Eastern Australian? It is hundreds of tiny things combined.
However, there are certainly some things that stand out as important. For example, the pronunciation of vowels. Tiny variations can make a big difference. I work with many people originally from India. One thing I hear a lot from them is the emphasis is different. For example, "The shipping maniFEST is wrong." (Caps indicate the emphasized syllable.) Tiny word choices also make a difference, even if they are correct, they might not be idiomatic. For example, you can usually tell an Eastern European from listening to their use of the crazy English definite article. Even the very best, most fluent have problems with this.
Unfortunately, using video chat amplifies the problem, because the slight visual jitter and clipped phonics itself gives a slightly "off" feeling to the conversation to start with even with native speakers, and so when slight pronunciation problems are added on top, it makes it even more noticeable.
Unfortunately, I think the only way to get really, really fluent, where your accent is indistinguishable, is to spend a lot of time talking with native speakers, ideally by living in the country where that is the native language. If that is not possible, perhaps you can get together regularly with a bunch of native speakers. Many of the accent problems come from slight differences in the structure of your native language, which bleed into your English. So unfortunately speaking with other learners from the same language group as you can mean you feed off each others problems.
However, FWIW, your written English is very clear, and that, after all, is the ultimate purpose of language.
Solution 3:
It depends on what you mean by 'foreigner'. I'm English, so all other native English speakers in the world are foreigners, including your US Skype friends :) In this context, I think you mean 'non-native English speaker'.
Apart from an accent, I'd have thought that a good grasp of grammar is the best way to change the way that your ability is perceived. I know non-native speakers who speak better English that I do - they may not sound like they're from the UK or US or wherever, but it doesn't matter because they can communicate effectively which presumably is the whole point.