What are the words and phrases that the researcher should avoid to use when writing a scientific paper?also, what points should a researcher put in mind when writing?
Qasim - academic authors should 'automatically' write in a style that is objective, neutral, unbiased, trustworthy and balanced. Words are phrases that are counter to that should be avoided i.e. biased, emotional, emotive, subjective, leading, untruths - and, of course, words and phrases that are offensive to anyone from any given race, creed, culture, religion, gender, sexual orientation etc. Sometimes, but not often, there are exceptions - such as topics that explore highly emotive contexts and the language (narrative) used is that of participants - such as qualitative quotation.
The page Osama Rahil shared and Dean Whitehood's advice are both on target.
I would add to avoid language that is overly self-promotional. Some writers are inclined to sing the praises of their methodology and findings, over-stating or over-promising the outcomes.
To add to the above, try to use active voice where applicable as much as possible. This means, if a point you are raising in your writing has been validated either by your own research or from research conducted by others (of course, sound methodology, analysis, and inference are a must in this regard), then say so concisely and clearly. Avoid flowery language and embellishment. State your case concisely, but comprehensively, using simple language. Where there is room for further investigation, point this out as well. Always remember to acknowledge limitations in your work.