Why when saying "I know" it's written 'Je sais' and not 'Je sait'?
The verb 'savoir' (knowing) needs to be in accordance with the 'sujet' (subject), which could be of 1st, 2nd or 3rd person. As 'je' (I, myself, then 1st person) is the subject, the verb 'savoir' should be expressed with a 's' at the end ('sais'). This rule is driven by the subject, and not the verb. Therefore, there is a bunch of french verbs accorded that way. Note that, by looking at the error of your question, the verb is accorded as if you were using it at the 3rd person 'il/elle' (he/she), which requires a 't' at the end.
I would like to build a new database in the company I am working for. I don't know where to start and need advice on ideal steps to follow to propose a great design for making a difference.
First, I think you should make a good assessment of business requirement with a good understand of business objectives, impacts and efforts in order to get there. With this you'll have a foundation for a minimal expression of a business case. Moreover, you'll be well informed to know which systems and databases that are required. From there, you should list data tables and design a design that provides ideal outputs to feed the technological stack. In an agile manner, you could build a prototype with technologies as MS Access for making a proof-of-concept in order to influence stakeholders, but also IT folks as your evolving prototype could be presented as specification reducing uncertainties, hence risks and costs.
For doing data science work, should I use an Hadoop-based solution to store and process information from a specific data source?
The answer depends on various factors. Which approach are you (and/or your team) using for data science? Which size your data is? Hadoop is a great approach for scalability and costs, but there is other options out there that could improve your "time-to-market" (as AWS, Azure, etc.). Also, it is better if you think abstract and be technologically agnostic for flexibility reasons over technology options and evolution of these in time.