When reading a paper few errors are as jarring as a misplaced or missing “the”. This particular error is very obvious to a native English speaker, yet is problematic for writers who's first language is not English. The reason is that the grammar rules governing the use of definite and indefinite articles ('the' and 'a', respectively) can change from one language to the next. In English, all nouns must be accompanied by an article, either 'the' or 'a'. 'The' is used whenever you are referring to a specific item, 'a' is used when you are referring to the item more generally.
Here are a couple of examples to help you if you're struggling to figure out where the “the” goes.
1. 'The' is always used when referring to a common noun – e.g. the protein, the equation, the network analyser.
2. 'The' is not used when referring to a proper noun – e.g. New York, Enrico Fermi.
The more you practice with definite articles the more natural placing them correctly becomes, and the more fluently your papers will read.