اُعْبُدُوا اللَّهَ شُكْرًا
Worship Allah out of gratitude.
- the commanded action is اُعْبُدُوا;
- اللَّهَ is the maf'ul bihi of worship;
- شُكْرًا gives the reason for worship: gratitude.
Because it is a mansub verbal noun expressing the reason, شُكْرًا is maf'ul lahu.