phrases - How do you differentiate between "in order to", "so as to", "so that" and "to"?

When we use the phrases so as to, in order to, and so that, we simply mean with the aim or
purpose of doing something. The first two phrases are always followed by an infinitive to.

Will I not be altering the meaning of the sentence, if I put both phrases in order to/so as to in one sentence, and also the phrase so that separately in another sentence? As in:

1. We took off our shoes so as to/in order to avoid scratching the newly finished floors.




2. We took off our shoes so that we can avoid scratching the newly finished floors.

Explicitly asking, which one of these phrases are informal and which one, not?

And is it okay to simply put the to-infinitive in written exams?

3. We took off our shoes to avoid scratching the newly finished floors.

Answer

Laboring under the distressingly common misapprehension that longer phrases
are inherently “more formal” than shorter ones, many novice writers,
especially budding technical writers rather than polished writers of
professional essays and fiction, will reflexively gravitate towards ever
longer and more sententious wordings.

At which point their copyeditors, should they be so lucky as to have such,
will equally reflexively rewrite those “wordy” constructions into simpler
ones. For example:

1. so as to ᴠᴇʀʙ > to ᴠᴇʀʙ


2. in order to ᴠᴇʀʙ > to ᴠᴇʀʙ


3. so that > so

Note that this example of yours:

We took off our shoes so that we *can avoid scratching the newly finished floors.

Has mismatched tenses: took is past tense but can is present tense. A happier sequence of tenses there would instead be the matched version shown here:

We took off our shoes so that we could avoid scratching the newly finished floors.

But as you suspect, there’s no reason to use so heavy a wording when a lighter one will do just fine:

• We took off our shoes to avoid scratching the newly finished floors.


• We took our shoes off to avoid scratching the newly finished floors.