How to tell the difference between the two meanings of "dream" in a sentence?

For example, "the girl I saw in my dream today last night was very beautiful", I think it's pretty clear what type of dream that is, but people still get confused.

English is very dependent on context. From the context above, it is clear what type of dream you meant - if people get confused, you should speak to them in their own language as, obviously, their English is poor.

Dream has two meanings:

OED

1. A. n.2 1.a. A series of images, thoughts, and emotions, often with a story-like quality, generated by mental activity during sleep; the state in which this occurs. Also: a prophetic or supernatural vision experienced when either awake or asleep.

2007 New Yorker 21 May 31/3 Last night, I had a dream that there was a snow snake coming down from a snow mountain to kill me.

3.a. A vision or hope for the future; (in early use chiefly) a vain hope or idle fantasy; (now also) an ideal, goal, ambition, or aspiration.

2010 Asian Woman No. 43. 15/2 Get that job, get that man (or woman), achieve that dream.

The solution would be to use another word for "dream" in sense 3a.

As a side note: the word "dream" in sense 3a is now somewhat dated, old-fashioned, or over-used - a cliché.

Not only is English dependent on context, as Greybeard has answered, but there are also stock phrases which are used more often than others.

We dream about things; we don't usually see things in dreams. Thus

The girl I dreamt about last night was very beautiful.

Seeing something in a dream may well connote the "goal" meaning of dream. Certainly a general mention of "your dream" would definitely refer to a goal. That's why people respond with "Is having a girlfriend your dream?", instead of "Do you dream about having a girlfriend?"

There's also another meaning, "ideal, perfect": "What would your dream holiday be?" That does not mean "What sort of holiday do you dream about?" or even "What sort of holiday is your ultimate aim to have before you die?" (because it might be entirely unattainable).

All of these meanings are related, and the way those different meanings are disambiguated is by using different expressions.

• What sort of holiday did you dream about?

  — What was the dream you had while you were asleep?

• Is having a holiday your dream?

  — Do you aspire to having a holiday?

• What would your dream holiday be?

  — What would be your perfect holiday if money and opportunity were no object?

The reason people get confused about "The girl I saw in my dream" is because that expression doesn't fit the common usages of dream.

Original: "the girl I saw in my dream today was very beautiful"

Some changes

The girl (that I saw) (in my dream) (today) was (very) beautiful.

I saw a girl (in my dream) (today) and she was beautiful.

1. "in my dream" modifies "saw" : I saw in my dream a girl.

2. "in my dream" modifies "a girl" : I saw a girl in my dream(I saw a girl who is in my dream).

People will generally say like this: I saw a girl of my dreams.

Now your sentence: The girl you saw in your dream + today

3. "in my dream" modifies the whole sentence : In my dream, I saw a girl or I saw a girl, in my dream.

Original: The girl [(that I saw in my dream) (today)] was very beautiful.

For your information, "in my dream" is close to "saw" and an adverb phrase which can modify verbs frequently. "today" is an adverb which can modify verbs and adverbs frequently and close to "in my dream"

So I think your original sentence is very clear to me.