1. His age is 14.
2. His height is 158cm tall.
3. His weight is 48kg.
(Are the sentences all grammatical?)
4. What is the negative sentence of the declarative sentence? When you make a negative sentence, you should put 'don't' before a base verb.
(Is this one grammatical? Would you check it? Thank you.)
1 and 3 are fine.
In 2, you don't need "tall" because the word "height" covers it.
4 looks good except that the word "sentence" is repeated. You can simply remove the first instance of it, or you can write something like this:
How do you make a negative in a declarative sentence?
The sentences you provided are mostly grammatically correct, with a few minor adjustments needed. Here are the corrected versions:
1. His age is 14. (This sentence is grammatically correct.)
2. He is 158cm tall. (This sentence rephrases the height information as an attribute of the person.)
3. He weighs 48kg. (This sentence uses the verb "weighs" instead of "weight" to convey the person's weight.)
Regarding your fourth question, the sentence "What is the negative sentence of the declarative sentence? When you make a negative sentence, you should put 'don't' before a base verb" is grammatically correct. However, it seems to be more of an instruction or a request for information rather than a declarative statement. To transform it into a declarative sentence, you could rephrase it as "The negative form of a declarative sentence requires 'don't' to be placed before a base verb."
If you would like to confirm the grammaticality of a sentence, you can provide the sentence, and I'll be happy to check it for you.