I encounter such a confusing sentence:
You are dealt two cards.
I don't understand why we should use "dealt" rather than "dealing"(Present Progressive Tense) here? What is the normal tense of "dealt" here?
In this context, you can assume that to deal = to give. So your sentence transforms into:
You are given two cards.
It means that while you are playing cards, you receive two cards (from the person holding the deck of cards).
It is also correct to say:
You are dealing / giving two cards.
but the meaning is different. In this sentence, you have the deck of cards, and you deal / give / share them with the other players.
are dealt => passive voice
are dealing => active voice
If you understand infinitive by "normal tense", then the infinitive of "dealt" is "to deal".
You have come across a passive sentence construction. Normally in an active sentence, the subject is what's called the agent. It is the one doing the action of the verb. The patient has the action applied to it. This is normally the direct object. This sentence also has an indirect object. The indirect object is often the receiver (or in this case the recipient) of the action, but doesn't experience the action. "You" are not being dealt, the "two cards" are.
In a passive sentence the agent is either missing and assumed, or included by adding by, then the agent, e.g. by the dealer.
In all the below sentences:
Here is a breakdown of possible forms. They all mean much the same thing, with different emphasis. The subject, direct object, and indirect object are not always the same from sentence to sentence, but the agent, patient, and recipient all are.
You are dealt one card.
You are dealt one card by the dealer.
One card is dealt to you.
One card is dealt to you by the dealer.
The dealer deals you one card.
The dealer deals one card to you.
The dealer is dealing you one card.
The dealer is dealing one card to you.
So, when the sentence uses is dealing, it is present progressive, as you observed in your question. When the sentence uses are dealt, it is passive, and only optionally includes the agent that is causing the action.
Being a native speaker of German, I'd answer this question thus:
When you have a sentence in the active voice, "Abel deals two cards to you", and you turn it to the passive voice, the direct object, "cards", becomes the subject: "Two cards are dealt to you". In German (and other languages that I know, and apparently in yours as well) this is the only way to use the passive voice.
English, hovever, can say "Abel deals you two cards", and, I feel, therefore loses the exact distinction between direct and indirect object. Therefore English can use the indirect object as the passive-voice sentence's subject, thus: "You are dealt two cards".
In English, the notion of "X [verb]s Y" can be expressed using the passive voice as "Z is [verb]ed by X". Unlike the active-voice construct where X must be specified, in the passive voice X may be (and often is) omitted, making the construct "Y is [verb]ed".
This construct can be applied to verb phrases as well as simple verbs, as in your example. Thus "You are dealt two cards" is equivalent to "You are dealt two cards by some unspecified entity", which is in turn equivalent to "Some unspecified entity deals two cards to you".
Note that if it were necessary to use the passive voice in the present progressive tense, the usage would be "You are being dealt cards" rather than "You are dealing cards". The past tense would be "You were dealt cards", the perfect tense "You had been dealt cards", and the imperfect tense "You were being dealt cards".