Question: For example:
You are talking to a person who has worked at a company for many years and you want to compliment him by saying "You must be very experienced" or "You must be very talented" or "You must be an expert after all these years"

Google translate gives the following:

> 
> Debe ser muy experimentado  
> 
>  Debe ser muy talentoso  
> 
>  Debe ser un experto después de todos estos años
> 
> 
> 

But as I understand it, *debe* means "ought to" or "should". So the phrase is no longer a compliment. It becomes a command.

I've searched around and I've seen people use "tiene que" in place of "Deber". But as I understand it, "tiene que" means "have to", so this changes the compliment to a requirement.

How can you say the phrases as a compliment?

Answer: 
This is very unusual but I agree with *Google Translate* this once on the translation but not in what you want to accomplish.

First let me say that I understand the "debe de ser" use as explained in @fedorqui's answer but in Colombia we many times omit "de" and just say "debe ser muy experimentado". This part is where I agree with *Translate*.

However I'd say that to another person and not directly to the subject especially in a speculative way. i.e talking to his boss or some other coworker I'd say:

> 
> Creo que él **debe tener mucha experiencia** después de todos estos años de trabajo.
> 
> 
> Creo que él **debe ser muy experimentado** después de todos estos años de trabajo.
> 
> 
> 

This is a compliment to the subject but the sentence is directed to someone else.

To say it directly to the subject I would not use the speculative sentence but instead an affirmative one without **debe**.

> 
> Tienes mucha experiencia después de esos años de trabajo. (You are very experienced after all those years of working)
> 
> 
> 

This last one I think is more of a compliment than the other because I'm stating the fact, while using "debe" or "debe de" could have all the other meanings you described in your question.

However since the tile of your question is "using *You must be* in the context of a compliment" I'd say it like a speculative/question.

> 
> Creo que debes ser muy experimentado después de todos esos años de trabajo. ¿Cierto? - I think you must be very experienced after all these years of working. Aren't you?
> 
> 
> Creo que debes tener mucha experiencia después de todos esos años de trabajo. ¿Cierto? - I think you must have a lot of experience after all these years of working. Don't you?
> 
> 
> 

I feel like saying it in question form is more polite and would make a better compliment which is what you want to accomplish.
The suggestions of Google translate are almost fine -they just need *de* after deber-; also "tiene que" would be correct. Why? Because it uses *deber*'s last meaning:

> 
> **[deber](http://dle.rae.es/?id=Bu2rLyz%7CBu8i6DA)**  
> 
>  Del lat. debēre.  
> 
>  (...)  
> 
>  6. tr. U. como auxiliar en las perífrasis, en las que añade una nota de inseguridad o probabilidad al verbo principal. *Debe DE hacer frío. Debieron DE salir a pelear.*
> 
> 
> 

Which is also explained by the Diccionario panhispánico de dudas:

> 
> [**deber**](http://lema.rae.es/dpd/?key=deber)
> 
> 
> (...)
> 
> 
> \2. Funciona como auxiliar en perífrasis de infinitivo que denotan obligación y suposición o probabilidad:
> 
> 
> (...) 
> 
> 
> b) deber de + infinitivo. Denota probabilidad o suposición: «*No se
>  oye nada de ruido en la casa. Los viejos deben de haber salido*»
>  (Mañas Kronen [Esp. 1994]). No obstante, con este sentido, la lengua
>  culta admite también el uso sin preposición: «*Marianita, su hija,
>  debe tener unos veinte años*» (VLlosa Fiesta [Perú 2000]).
> 
> 
> 

That is, it uses the construction *deber de* to indicate *must be*. All together, your sentences would be:

> 
> Debe de ser muy experimentado
> 
> 
> Debe de ser muy talentoso
> 
> 
> Debe de ser un experto después de todos estos años
> 
> 
>