1 Introduction  
2 Multi-task Training Methods  
2.1 Encoder-Decoder Architecture and Two Multi-Task Training Frameworks  
2.2 How Does MTL Work  
3 Training Methods: Applications  
3.1 Joint Training Applications  
3.2 Multi-step Training: Applications  
4 NLP Task Relatedness  
4.1 Joint Training  
4.2 Multi-step Training  
5 Future Directions  
5.1 Regarding Task Relatedness  
5.2 Regarding Training Methods  
6 Conclusions