Use of present tense vs future tense

Question

Because I feel that I am going to say so I always use following line:

"I will say/suggest that you must not go there."

Is above line correct? or, should I use following line:

"I say/suggest that you must not go there."

But I think that this line has connotation that I say something regularly or frequently.

Answer 1

You usage of the phrases "I will say/suggest" or "I say/suggest" is called a hedge. It is an additional phrase that you use to somewhat soften the message.

In the following, the parentheses include an unspoken, implied message, that you may want to make.

Assuming you want to use the word "must", the strongest message you could make would be

You must not go there. (This is a command!)

This may sound like a command, and often you don't want to make that impression. Adding "I say" (or "I say that") creates a slight "way out".

I say you must not go there. (But someone else may have a different opinion)

This "I say" is idiomatic and does not mean you say it regularly (as simple present usually means).

Further "softening" can be achieved by using "I would say".

I would say you must not go there. (I would, if you asked me, but maybe you don't ask me. So I didn't actually say that.)

Using "will" creates a different effect; it stresses the statement (usually with audible emphasis on "will") when this is a part of a longer sentence.

Although I understand your desire, I will say you must not go there. (I don't want to hurt your feelings but I really mean it)

Using "I suggest" instead of "I say" is an explicitly softer message. You can't use "suggest" with "must" - it would be a contraindication. "Suggest" is followed by "that" and a statement in the present tense. As stated in Nick's answer, it should be in the subjunctive mood, but in informal English a regular (indicative) statement is commonly used.

I suggest that you not go there. _{(subjunctive)}

or

I suggest that you don't go there. _(indicative)

Answer 2

Well, first off, if you're going to use the verb "suggest", you should use the present subjunctive:

I suggest that you not go there.

If you're going to use the verb "say", you can say it the way you have it written, but, to me, it sounds awkward or strange:

I say that you must not go there. (strange construction, but it's grammatically correct)

I would say it this way:

I say that you should not go there.

I say that you shall not go there.

I say that you ought not to go there.

The best way to say this is by using the verb "suggest" with the present subjunctive as I have stated above. I also think that you could just drop the "I say that" or the "I suggest that" clause and just write it like this:

"You must not go there."

The clause "I say that" or "I suggest that" is superfluous if you just want to tell the person that he must not go there.

If you want to tell someone where he must not go by using the future "I will say that" or "I will suggest that", then that's fine, but it's not necessary per se:

I will suggest that you not go there. (It's fine, albeit unnecessary)

I will say that you must not go there. (It's fine, albeit unnecessary)

It is my personal opinion that my examples above are more in line with how native speakers would say it. They correct some of your odd constructions, but I would not say that your constructions are not grammatical or idiomatic; I would just say that they are not as common as the ones I've written above for you; and for all intents and purposes, the ones I've written for you mean the same thing even though a few of them may have slight gradients in meaning; nevertheless, you would still get your message across to the reader.

I hope this might have helped you out. Take care and good luck!