I would like to know what is the most obstacle to your spoken english? Pronunciation, grammar, vocabulary, or something else?

Pronunciation: sometimes others do not know what you are talking about, cause you have a bad pronunciation, wrong word stress.

Grammar: probable your pronunciation is good, but when you are speaking, you feel nervious, always make some grammar mistakes, select unsuitable words. That makes you are not a perfect english speaker. It is difficult for you improve it, cause others will not tell you your spoken mistakes.

Vocabulary: When you talk about something, you always can not find a word in your mind to express it.

So what do you guys think is the most important?

To me, I always make some grammar mistakes.
I would like to know what is the most obstacle to your spoken english? 'greatest'

I am somewhat proficient in three languages other than English. My biggest problem in conversations, of course, is understanding the other people, usually because they speak the language faster than I can think in it. So, I usually start any conversation more slowly that I really need to, and that seems to slow them down a bit. Vocabulary can also add to my problems.

As for people learning English, I find that the biggest problem I have with them is their attempt to speak faster than they really should, thus making it difficult for me to comprehend. I believe that many Asian languages use far more vowels than consonants, and when speakers of Thai, for example, speak English fast, they often do not give as much attention to the consonants in English. On the other hand, an English speaker will make many (sometimes embarrassing) mistakes because of the tonal system in Thai.

Each language-to-language situation is different, depending on the languages involved; these are problems that a language learner should be aware of and try to get help from a person thoroughly familiar with both languages.
Pronunciation is very important, but with English that means stress patterns. Getting the stress patterns right for a whole phrase is often more important than getting every single word exactly right. The vowels can be off a bit, but the stress pattern and the consonants should be as accurate as you can make them. And be aware that spelling does not always indicate pronunciation very well in English! You may have to force yourself to overcome the habit of reading English with the same correspondences to sounds that you find in your own language.

Next is words, words, words. You cannot speak without words! Learn as many words as you can. Learn frequently used phrases as units. Practice saying phrases with the correct stress patterns. You can learn a lot of words without too much effort by reading a lot.

Grammar comes somewhat automatically for some people, especially if they are good at listening and finding word patterns and imitating the structures. Others need to study it in a more rigorous way to feel confident about it. Choose whatever method works best for you.

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As for me the biggest obstacles in learning English are mainly the other languages that I speak, I mix them up aaaall the time. Especially French is very unhealthy to my English grammar;-) because a lot of words are written the same way and others are not, such as endings like -ance, ence etc. That's really tricky to me. And those darned prepositions, I often don't know if it's correct to use "for","to" or "of" in certain situations since in my mother tongue their usage differs a lot. I guess the best thing we can do to overcome these obstacles is simply to keep going and trying and practicing as much as we can, and not to be afraid of making mistakes.
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From a learner perspectives, spoken form of English is a lot more difficult to master than written. The reason is obvious. We often don’t have the time to compose a perfectly constructed sentence or response whereas in written form, we can search in our vocabulary for the proper words and the best construction. That’s not the only obstacle. In order to achieve to a fluent level of spoken English, we have to pronounce every word clearly and develop a decent vocabulary as well as control of intonation, tone, inflection, and stress. This process take a long time to perfect which requires patience and strong wills. My experience was just keep practicing. Learn the correct speech patterns and expand your vocabulary through audio-visual sources such as newspapers, magazines, television and radio programs.
In the beginning everything is difficult, I don't think any aspect of a language is more difficult than another. I think vocabulary in English possibly presents the biggest problem for learners given its vastness. Not knowing perfect grammar rarely impedes communication and like most things once one gets better at the language all aspects including the grammar fall into place. Pronunciation too is tricky, especially in the beginning, but that too comes with time. And when you listen to native English speakers, they make 'mistakes' too in spoken conversation, conversations go back and forth, switch direction and so on. Even in writing, native speakers have to be taught grammar and often make mistakes.

I think when learning, it's important to have fun and enjoy the language. Don't get too hung up on perfection.