Are these well written sentences?

1. When I am writing, my words need to construct a clear picture for my readers.

2. I will avoid using slang in my writing.

3. Using irrelavant words, is one error I need to change in my writing.

4. I need to use a dictionary while I am writing, so that I can look up unfamiliar words.

5. It's important that I study the meanings and spellings of words that are confusing to me.

Only one of the three commas you used is needed. Do you know which one?

All else is fine.

4. I need to use a dictionary while I am writing, so that I can look up unfamiliar words.

No.

The comma in #1 is correct. The others are not.

Thank you! I just caught that when I was rewriting that sentence.

I am still confused about the comma here. I thought you use a comma before for, and, or, so, but, nor, and yet?

4. I need to use a dictionary while I am writing, so that I can look up unfamiliar words.

... but not before "so that" -- when the conjunction is one of the FANBOYS, then the clauses are both independent, and a comma and COORDINATING conjunction is needed.

However, "so that" is a SUBORDINATING conjunction -- a very different thing.
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions

Okay, I get it now. Thank you for your help.

Oh, good! You're very welcome.

As an AI, I can analyze these sentences for you. Here's my evaluation:

1. When I am writing, my words need to construct a clear picture for my readers.
- This sentence is well-written. It conveys a clear message and uses appropriate vocabulary.

2. I will avoid using slang in my writing.
- This sentence is well-written. It states a clear intention and uses proper grammar and vocabulary.

3. Using irrelevant words is one error I need to change in my writing.
- This sentence can be improved. It appears to be missing punctuation after "words." You can add a comma after "words" to separate the sentence's clauses properly.

4. I need to use a dictionary while I am writing so that I can look up unfamiliar words.
- This sentence is well-written. It expresses a clear idea, and the use of "so" instead of "so that" makes the sentence more concise.

5. It's important that I study the meanings and spellings of words that are confusing to me.
- This sentence is well-written. It accurately conveys the importance of studying meanings and spellings of confusing words.

To answer your initial question, sentences 1, 2, 4, and 5 are well written. Sentence 3 can be improved by adding a comma after "words."