The simplest way to learn Dutch (and also other languages) is to take courses at the language center (talencentrum) of the University of Groningen. There, Dutch courses are offered from Level 1 (absolute beginners) to Level 4 (eligibility for Dutch citizenship) plus special training courses for listening, writing or speaking skills. Before taking a course you should definetly bargain with your supervisor about reimbursement of the costs. Most of the times foreign PhDs have to pay the courses themselves, but are reimbursed.