Where is the difference between below sentences and which one of them is the correct usage to express "strong internal necessity or Inner passion" and in which tense (time) should I use each of them?
For example, which one of the below sentences is true:
I have to say to you, you are a very good person.
I shall say to you, you are a very good person.
I should say to you, you are a very good person.
I must say to you, you are a very good person.
By most modern understandings, have to and must imply compulsion, should implies obligation without compulsion, and shall implies firm intention or commitment - or is just used in place of will to lend a certain air to the text or speech.
In contracts, standard documentation and various other specialised usages have their own understandings of the term. If you need to understand them legally, you should speak to a lawyer. I know in some cases shall is a stronger term than must, and in others it is the only acceptable thing because the document is describing what each party is committing to do.
To reflect "internal necessity or inner passion", I would say that you are talking about a compulsion, so "have to" or "must" are both appropriate. They are both (effective) modal verbs, and as such can theoretically be used in any tense, though not all modals exist properly in all tenses; "must" is often replaced with "had to" to express compulsion in the past or with "have to" to express compulsion in the future. They are also both used in front of the have of the perfect tense in order to indicate inference in the past, just to be clear.
I have to park there.
I must park there.
You have no choice but to park there, or are under some strong compulsion to do so. This is often used in a hyperbolic manner, to express a strong need or desire to park there (or whatever the verb is), without there being literal compulsion.
I had to park there.
This means you had no choice but to park there.
I have to have parked there.
I must have parked there.
This means that you may not remember that you parked there, but all the evidence suggests that you did park there.
I had to have parked there.
You may not remember that you had parked there some time in the past, but the evidence suggests that you did.
Using them about the future restricts you to have to rather than must, unless you use the futurate (the use of the present tense to talk about the future).
I will have to go to school tomorrow.
That's the future tense with the obligation/compulsion.
I have to go to school tomorrow.
I must go to school tomorrow.
That's the futurate.
"I shall" is just an alternative way of expressing the future (= "I will"). Normally, "shall" carries a sense of modal obligation only in the second and third persons ("you shall", "he/she/it shall") - and not always then. (The first-person interrogative "shall I?" does have a modal sense: it's used to ask for advice or suggestions about the appropriate the course of action, or to offer to help. "Should I?" is sometimes used the same way, other times with a slightly stronger sense.)
"I should" normally carries an implication of duty or obligation (although sometimes this is absent in the first person in BrE, where "I should imagine" can mean "I would imagine", and "I shouldn't do that" can mean "I wouldn't do that"). But the sense of duty or obligation expressed by "should" is milder than that of "have to" or "must". In fact, quite often there is a spoken or unspoken "but" ("I should return the book, but I haven't read it yet" or "I should apologise to him, but I don't want to"). "I ought (to)" is similar to "I should" in expressing a sense of mild duty or obligation.
"I have to" and "I must" express strong obligation or necessity. "I must" is stronger. According to Cambridge:
‘Must’ is also used to talk about what is necessary, though it can sound urgent in a way that ‘have to’ and ‘need to’ generally do not
In short, the sentence you're looking for is either "I have to say, you are a very good person" or "I must say, you are a very good person" - although "I should say" would be OK too.
Either "I have to say" or "I have to tell you" would be more idiomatic than "I have to say to you", although "say to you" is correct as well.
