The original sentence is:
Ich schreibe heute einen Brief.
I know it is okay to say:
Heute schreibe ich einen Brief.
But is it okay to say:
Einen Brief schreibe ich heute.
If it is okay, is it common?
The original sentence is:
Ich schreibe heute einen Brief.
I know it is okay to say:
Heute schreibe ich einen Brief.
But is it okay to say:
Einen Brief schreibe ich heute.
If it is okay, is it common?
All three are ok. The first one is the most common.
The second one emphasises "Heute". It would be a common way to answer the question "Was machst Du heute?"
The third one is the most uncommon. It emphasises "Einen Brief". Maybe you would use it to answer the quesion "Was schreibst Du heute?" - but the most common answer would be simply "Einen Brief."
Edit: As Felix Dombek pointed out in the comments, the third example could also emphasise the word "Einen". So it could be the answer to "Wie viele Briefe schreibst Du heute?" - "Einen Brief schreibe ich heute."
Word order is often used for emphasis where the first to come usually is the one you put emphasis on:
- Ich schreibe heute einen Brief. Du schreibst heute einen Brief. Maria schreibt heute einen Brief.
- Heute schreibe ich einen Brief. Morgen schreibe ich einen Brief. Diesen Monat schreibe ich einen Brief.
- Einen Brief schreibe ich heute. Ein Buch schreibe ich heute. Eine Notiz schreibe ich heute.
- Einen Brief schreibe ich heute. Vier Briefe schreibe ich heute. Keinen Brief habe ich heute geschrieben.
Here is an everyday dialogue where your "Einen Brief schreibe ich heute" could appear.
[A and B chatting next to a construction site with 5 jackhammers working simultaneously]
A: Heute schreibe ich einen Brief!
B: Was?
A: Heute schreibe ich einen Brief!
B: Einen was [shouting] schreibst du heute?
A: Einen Brief [shouting] schreibe ich heute!
So, I would say: yes, that sentence could appear in real life, but you have to go to the extremes to find a situation where it really would.
Although a different order, the emphasis of
Ich schreibe heute einen Brief.
and
Heute schreibe ich einen Brief.
is similiar, which is that you simply want to communicate that you write a letter today. Both are really common phrases, however the third option, which is
Einen Brief schreibe ich heute.
is very uncommon and cannot be used simply as an alternative to the first two options. The emphasis is clearly on "Einen" (one) and it could be used like in the following example:
"Ich muss sehr viel Geschäftskorrespondenz in dieser Woche verfassen. Einen Brief schreibe ich heute. Die anderen dann in der restlichen Woche."
In Englisch:
"I have to compose a lot of business correspondence this week. One letter I write today. The others the rest of the week."
I agree with the other answers that the word order determines the intonation of a sentence; however, not in the way those answers claim. Which words are being emphasized is dependent on the context.
Most answers suggest that Ich would be emphasized; but consider the following dialog, in which the asker is interested in an entire act:
A: Was machst du (heute) so?
B: Ich schreibe heute einen Brief.
It is more natural to emphasize schreibe and Brief, because these words comprise the relevant information. No word can be dropped, which is different in the fllowing situation:
A :Wer schreibt heute einen Brief?
B: Ich.
Of course, the respondent could have also repeated everything:
B: Ich schreibe heute einen Brief.
Here, the emphasis would be as suggested by others. But I consider it unusual that the irrelevant words are repeated.
Most answers suggest that Heute would be emphasized; but consider the following dialog, in which the asker is interested in what you do today:
A: Was machst du heute?
B: Heute schreibe ich einen Brief.
Or, more simply:
B: Einen Brief schreiben.
Here, the relevant information is Brief. When the asker is interested in when something is done, the answer would certainly be simpler:
A :Wann schreibst du einen Brief?
B: Heute.
Again, the respondent could have also repeated everything:
B: Heute schreibe ich einen Brief.
But the repetition of the irrelevant words is unusual.
The intonation of this sentence is maybe the most controversial one. The other answers suggest that Einen Brief would be emphasized. Indeed, a possible dialog could be:
A: Was schreibst du heute?
B: Einen Brief (schreibe ich heute).
But the words in parantheses are irrelevant. There is another possible way of emphasis:
A: Und wann schreibst du mal einen Brief?
B: Einen Brief schreibe ich heute.
or simply
B: Heute.