Abstract is very important for a paper. Readers may first read the abstract and then decide whether they'll continue to read the whole paper.

When we invite the reviewers, we only send the abstract to them and wait for their reply. If the reviewers read a badly-written abstract, then he will directly decline the review invitation, and some of them will write to the editors,

"after a quick look at the abstract, I think this paper has no novelty. "

"The absract makes me stop looking down for the rest of this paper. "

But abstract is very difficult to write well.

Abstract is the mini-version of the fulltext. It should inlcude the research objective, materials and methods, the major and important results, and then a conclusion.  Authors should first write the full text and then write the abstract.

Jargons should be avoided. Reference should not to be included. Acronyms should only follow the fullname or be followed by the explanation. Equations or formulars are better to be avoided (they can be explained in words )

Should we use Active voice or Passive voice in the abstract? Try to use active voice if you can express well in active voice.

Should we use past tense or present tense in the abstract? Use past tense to descibe the experimental process and use present tense to descibe the methods and results.

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Here, I recommended one very useful Link.

Writing the Abstract

http://www2.plymouth.ac.uk/millbrook/rsources/litrev/lrabstract.htm