Cfrtjryk

A Brazilian friend speaks English very well, but has a very unique habit: it seems often that she needs to use "for" but she instead uses "to", and vice-versa.

For instance:

The present is to Thomas. (should be "for")

Say hello for your wife. (should be "to")

I have tried looking in the dictionary to specifically determine which definitions she's confusing, and it seems that she's mixing up using "to" as a consequence and "for" to indicate a purpose.

How can I help her find a way of remembering to use them correctly?

Edit: We recently ran into a better example:

You started working on a fix to that problem. (should be "for")

One possibility, understanding that prepositions have very slippery and often idiosyncratic meanings ... have her think of to as indicating a destination:

I sent the present to him.

I gave the present to her.

Whereas for can indicate or "for the good of":

I did it for her.

The present is for him.

Do you think this might help? Note that we're likely to find many cases where these simplistic definitions don't work, alas.

The problem is that both "for" and "to" translate to Portuguese in these cases as para.:

"The present is for Thomas." --> O presente é para o Thomas.

"Say hello to your wife." --> "Diga oi para a sua esposa."

As a native Portuguese speaker (I'm Brazilian too), I'd say that there's no simple rule of thumb to always avoid this confusion. You can explain to her what Mike Pope said in his answer; that should help. But it's only by listening and repeating the appropriate usage that we will naturally learn to use these prepositions correctly.

This is my advice for her... I mean, my advice to her.

I'm a spanish speaker and I sometimes confuse them too, because we use the word "para" with both meanings.

In this example "Say hello to your wife" and in all the sentences where you have "dative case" you should use "to" I think.

There's also the situation where either is acceptable.

I read a story to the children.

I read a story for the children.

Deeper, more profound answers require one to appeal to Linguistics. So I Googled "semantics of english prepositions" which revealed many references such as the following:

Applying Cognitive Linguistics to Learning the Semantics of
English
to, for
and
at:
An Experimental Investigation by Andrea Tyler, Charles Mueller, Vu Ho.

At 26 pages, it is too long to reproduce here, but the following quote from p 2 of 26 (Introduction) should already convince you of and to evidence its helpfulness.

  Language teachers and researchers have long recognized that the acquisition
of prepositions poses major challenges for second language learners (e.g., Celce-
Murcia and Larsen-Freeman, 1999). One reason for this is that the semantics of
prepositions are notoriously difficult to characterize. For instance, on first inspection,
the distinction between prepositions such as
over
and
above
is quite unclear. On one
hand, the sentence:
The picture is over the mantle,   is a near paraphrase of:  
The picture is
above the mantle.

On the other hand, the sentence:
Mary hung her jacket over the back
of the chair

is interpreted as meaning something quite different than:
Mary hung her
jacket above the back of the chair.

Additionally, prepositions tend to develop a complex
set of extended meanings, for instance,
over
has developed at least 16 meanings,
many of which do not appear to be systematically related. Although linguists have
long been aware that prepositions develop complex polysemy networks, the meaning
networks surrounding spatial markers (and the systematic processes of meaning
extension from which they result) have only become the foci of linguistic inquiry
in the last 20 years. Even the best descriptive grammars and dictionaries present
the multiple meanings of spatial language as largely arbitrary. Traditional accounts
have represented the semantics of English prepositions as arbitrary (Bloomfield, 1933;
Frank, 1972; Chomsky, 1995). Consequently, pedagogical treatments have often
suggested memorization as the best strategy. Studies show that accurate use of spatial
language is one of the last elements learned and many highly proficient L2 speakers
never attain native speaker-like use (e.g.,
Lam, 2009). Indeed, Lam found that L2
Spanish learners made virtually no gains in their mastery of the prepositions
por
and
para
over the course of four years of college Spanish.

   Cognitive Linguistics (CL) offers an alternative perspective, suggesting that
the many distinct meanings associated with a particular preposition are related in systematic, principled ways (e.g., Brugman, 1988; Dewell, 1994; Dirven, 1993; Lakoff,
1987; Linder, 1982; Hawkins, 1988; Herskovits, 1986, 1988; Tyler and Evans, 2001a,
2003; Vandeloise, 1991, 1994)

Ensure to consult and try the many References on pp 22-26, which includes (on page 25) the following which I plan to read myself:

Tyler, A. & Evans, V. 2003. The Semantics of English Prepositions: Spatial Scenes,
Embodied Meaning and Cognition.
Cambridge: Cambridge University Press.