For this meaning, “so” joins two complete sentences and shows the result of something in the second sentence. Take a listen:

The apartment was too hot, so we opened the window.

The first sentence provides the action or situation: The apartment was too hot. It is joined by “so” to the second sentence which explains the resulting action: opening the window.

In order to…

OK, now we move to “so that,” a phrase that begins adverb clauses. You may recall that an adverb clause shows a relationship between two actions. It joins a main clause, or complete sentence, to a dependent clause, or incomplete sentence, and shows how they relate.

“So that” means “in order to” which answers the question “Why?” We use it to begin adverb clauses of purpose. Let’s hear an example:

It helps to lower blood sugar so that you feel less hungry.

The adverb clause is “so that you feel less hungry.” It shows the purpose for the action in the main clause. Why does it help to lower blood sugar? To feel less hungry.

“So that” can also come at the beginning of a sentence but this is rarer and usually sounds stilted to Americans. Listen:

So that you feel less hungry, it helps to lower blood sugar.

One important note is that the word “that” is optional for “so that” in spoken English, so it may disappear. Here’s how that sounds:

It helps to lower blood sugar so you feel less hungry.

But without “that,” how will you know that the meaning is “in order to”? One signal is that there is often a modal verb in the adverb clause. Modal verbs include can, could, may, might, will and others. Here’s how that might sound:

It helps to lower blood sugar so you can feel less hungry.

The modal verb in the adverb clause is “can.”

To the level described…

Finally, let’s discuss the phrasing “so adjective/adverb that.” This is different from both meanings we’ve talked about. To try to put it simply, it means “to the level described.” Here are some examples:

This morning, the construction was so loud that we could not sleep.

This means that the construction was loud to a level that prevented us from sleeping.

In the phrase “so adjective/adverb that,” the word “that” begins a type of clause called a “complement clause," but we will not talk about that in today’s program.

What’s important today is knowing that the word “that” for this phrase is also optional and may disappear. Here’s how it sounds:

This morning, the construction was so loud we could not sleep.

So, how might you know the meaning if “that” disappears? Well, you can listen for an adjective after “so.” The adjective in our example is “loud.”

Comparing the three

Now, we’ll compare the three phrases. You will hear three sentences that sound similar but have different meanings:

He is funny, so his friends laugh at him.

The meaning here is: He is funny. Therefore, his friends laugh at him. Here’s the next one:

He makes jokes so that his friends will laugh.

This means: He makes jokes for the purpose of making his friends laugh. It answers the question, “Why does he make jokes?” Notice the modal “will” in the adverb clause.

Now for the last meaning:

He is so funny that his friends laugh at him.

It means: He is funny to such a level that it makes his friends laugh at him.

Now for one more set to help you examine the meanings:

She studied hard, so she was able to win a scholarship.

The meaning here is: She studied hard. Therefore, she was able to win a scholarship. And the next one:

She studied hard so that she could win a scholarship.

It means: She studied hard for the purpose of winning a scholarship. Again, you see a modal in the adverb clause: could. And lastly:

She studied so hard that she won a scholarship.

This means: She studied hard to such a degree that she won a scholarship.

You probably noticed that some examples other than those with “so that” have modal verbs. All three of today’s phrases can have modals, but “so that” adverb clauses commonly have them.