Generally speaking, learning to write in a foreign language demands you to go through three steps, although there is not clear distinction between them.
First, learning the basic vocabulary and grammar of this language. At this time, learners have not grasped the basic usage, therefore are still in the basic stage of language learning. Writing exercise usually does not exit in this period; even if it does, it mainly serves a kind of vocabulary or grammar exercise. This kind of writing practice presents very simple contents, but requires that learners should use both vocabulary and grammar correctly.
Second, learning both language and thought expression. At this time, learners have already got familiar with the basic vocabulary and grammar, and can express simple ideas in this language. Writing exercises at this stage can work for two purposes. Firstly, they can help learners further their understanding of this language. Secondly, they can help learners express certain ideas in foreign language. In the learning practice, these two aspects are actually inseparable. Many students start to keep diary in English from Junior Three; they are in fact progressing in this second stage.
Third, learning to focus on thought. When learners can deal with the vocabulary and grammar of a foreign satisfactorily, and can use the language to express rather complicated ideas, they start to enter the third stage. Here language is no longer the biggest problem; instead, how to present good contents gains top priority. What learners pay attention to is how to choose their materials well, how to organize the ideas well, etc. Some people hold that only this third stage is really writing. This may be too biased. Writers, however, have to reach the third stage in order to be successful.
If you find it difficult to write complete sentences, you are still in the first stage. At this stage, you do not have to rush to long articles, although you may have some precious profound ideas in your mind. You should make efforts in language accumulation, paying special attention to usages of common words and sentence patterns. Besides, doing translation exercise may also help you improve your ability in using the language, thus making preparation for improving your writing ability. Many exercises in our translation column are also helpful in improving your ability in writing. What's more, doing more reading exercises can also help sharpen your sensitivity towards the language.
To sum up, you may try to make efforts in these three aspects: accumulate language materials, try some translation exercises, and take up some reading exercises. As your language proficiency gradually improves, you will also gradually move into the second stage. Then you will have no trouble in producing complete sentences.