If you arrived in February 2014, you are still a nonresident as you have been in the US less than 5 years or part years ( 2014, 2015, 2016, 2017, 2018).
You start counting days of presence for the substantial presence test on January 1, 2019. As you will be in the US more than 183 days in 2019, your will be a US resident for tax purposes as of January 1, 2019.
You need to file Form 8843 for 2018 as a nonresident even if you have no tax filing requirement.
https://www.irs.gov/individuals/international-taxpayers/foreign-student-liability-for-social-securit...
