Generate sentences from keywords by adding formal word
I am also fairly new to this, but am working on the same type of problem where I have to predict the sentence based on previous data. Like you want giving X value predict y value.
So, basically you need to create a dataset as per your requirement and once your dataset is ready you can train your model with encoder-decoder or LSTM layers.
Please find below links that may be helpful to you to fulfill your requirements. See as X and Y value input output prediction.

Gmail style smart compose
Neural Machine Translation with Attention
Sentence Prediction using LSTMs