tense - Understanding the future perfect

We can use the future perfect to say of something that it will have been done by a certain time in the future. Does it make sense to use this tense to tell about something by an undefined time in the future? For instance:

I will have learnt English.

My understanding:

This means that I will have learnt English in the future. It is not defined when exactly I'll have learnt English. This sentence emphasizes a future achievement, not the learning process.

What exactly is the difference in that sentence from future prediction with simple future:

I will learn English.

Answer

Perfect constructions are relative tenses: the eventuality named by the lexical verb (learn in your example) is located at an Event Time (ET) prior to the Reference Time indicated by the tensed verb (will have in your example) in the construction. The entire construction describes a state at RT which arises out of the eventuality at ET.

Consequently, there must be a definite RT to which ET is related. However:

1. 
   

   When we say that the RT must be ‘definite’ we do not require a date/time measure such as tonight or in 2018. RT may also be defined as a particular event, or entry into a particular state. For instance:

   
   

   
   

   By the time I graduate I will have learnt English.

   
   

   
   

   As a rule of thumb you may think of ‘definite’ here as meaning “defined enough to recognize it when you see it”.

   
   


2. 
   

   RT does not have to be defined in the same sentence the perfect occurs in. It may be defined earlier in the discourse; all that is necessary is that your hearer must be able to identify what time you are talking about. For instance:

   
   

   
   
   

   I expect to graduate before I am 22 years old. By then I will have learnt English.

   
   

   Q: What will you have accomplished by the time you graduate?
   A: I will have learnt English.