The northwest of a place, country, or region is the part which is toward the northwest.
西北部 • ...in the extreme northwest of the country. …在该国的最西北部。
The northwest part of a place, country, or region is the part which is toward the northwest.
西北部的 • ...the northwest coast of the United States. …美国西北部海岸。
If you go northwest, you travel toward the northwest.
向西北 • Take the narrow lane going northwest parallel with the railway line. 走那条与铁路线平行的通向西北的窄路。