When considering tense, the first thing to note is that English is a very easy language. There are only two tenses - the present and the past. This is in the strict sense of 'tense' - that there is a change in the form of the verb to indicate a change in the grammatical treatment of relative time.
The second thing to notice is that English is a very difficult language. There are countless ways of showing more or less subtle ideas of time and mood, or tenses. This is in the loose sense of 'tense' - in English, collections of words to make verb phrases which substitute for more formal and regular patterns of changes in verb endings to show the same phenomena of grammatical marking of relationships in time.
When telling the story of something that has happened in the past, in academic English one must use the past tense -- the form of the verb that, in regular English verbs, ends in -ed. Say "Einstein published his Theory of Relativity"; "the rouble was devalued"; "the Treaty was signed", and not "Einstein publish
es his Theory"; "the rouble is devalued"; "the Treaty is signed". This use of the historic present is seen as good style in many languages, and even in some kinds of English. It is not encouraged in academic writing.
One exception to this is the way in which academic writers report other writers. It is common to say either "Shakespeare says [present tense] 'To be or not to be'" OR "Shakespeare said [past tense] 'To be or not to be'". These two are interchangeable. Both are acceptable. If there is a difference, it is a very small one, depending, it may be conjectured, on the state of mind of the writer: if you think Shakespeare's words remain true, vibrant and alive now then write in the present tense; if you are quoting them merely as a curious fact about what was thought or written 400 years ago, then write in the past tense.
See also may and might.