Is there any rule you think is valid that discourages the use of a certain word to start a sentence?
Because I suspect the answer is no.
But it would be good to have a blanket answer to this kind of question.
Well, with certain words it's simply impossible to start a grammatical sentence: one such word that comes to mind is "ago". It always comes after other words (e.g. "one hour ago"), never at the beginning of a sentence or clause.
[Before someone points it out: note the use-mention distinction. A sentence like
'Ago' is a word you cannot start a sentence with.
starts with the word "'ago'" and not with the word "ago".]
But if your question "Is there any valid rule discouraging the use of a certain word to start a sentence?" (emphasis mine) implicitly restricts attention to words that can grammatically start sentences, then it's not clear what it would take for a rule that discourages something grammatical to be "valid". Certainly there exist people who disapprove of certain words starting sentences for their own idiosyncratic reasons, such as the "but" I started the previous sentence with. Are these "rules" valid? I wouldn't consider them valid, but I don't know what valid means to you. :-)
In school we were taught to "never begin a sentence with 'and', 'but', or 'so'". But I do anyway. And so do a lot of other people.
The past tense of many verbs make no sense as the first word in a sentence: 'went', 'tried', etc., unless it's in a context where sentence fragments are OK: 'Where you been?', 'Went to the shops'. 'Ago' is a particularly nice example because it wouldn't even work in an informal sentence fragment.
Is there any valid rule discouraging the use of a certain word to start a sentence?
There's a rule (can't say if it is valid or not) that numbers, no matter how long, at the start of a sentence should be written out in words,
Seven thousand, four hundred and seventy-six trombones led the big parade
so some people avoid putting numbers there. Also, it's a bit odd to capitalize terms which are computer commands which won't work if they are capitalized:
Lsis how you get a listing of your files
so some people might avoid putting those terms at the start of sentences.
Use of greek letters or numerals is discouraged. See also Is it okay to start a sentence with a Greek letter (variable)?
Because is also another word that you should not start a sentence with.
Beginning sentences with conjunctions (and, or, but, etc.) is the same as using fillers (uh, um, er, etc.). They do not serve a purpose to the sentence. They can be removed without disturbing the meaning/purpose of the thought. Conjunctions connect things. They do not stand alone. Now, either people are too confused about that fact or they are caught up in the fact that they pause too much and at the wrong places in a sentence thought.
"Because I suspect the answer is no," should be "I suspect the answer is no."
"But it would be good to have a blanket answer to this kind of question," should be "It would be good to have a blanket answer to this kind of question."
You typed 3 sentences, which should have only been 2 sentences: a question and your answer.
" 1) Is there any rule you think is valid that discourages the use of a certain word to start a sentence? 2) Because I suspect the answer is no. 3) But it would be good to have a blanket answer to this kind of question," should be as follows:
1) Is there a valid rule that discourages the use of certain words at the beginning of a sentence?"
2)I suspect that the answer is no, but it would be good to have a blanket answer to this kind of question."