The difference between SPEAK and TALK

Thank you for your question. On August 3, 2015 I answered a question about the differences between say and tell (click here to see the answer), so here I will focus on the similarities and differences between speak and talk. 

Similarities between speak and talk

In general, both speak and talk are used to refer to the act of expressing thoughts with words. Speak and talk can be used interchangeably in sentences like these:

• I need to speak/talk to the manager here.
• You can speak/talk freely here. No one else is listening.
• Now it’s his turn to speak/talk.

The most common structure with either speak or talk is the verb(speak/talk), possibly followed by to/with + the listener, possibly followed by about + the topic of the speaking or talking. Here is that structure in short form:  

SPEAK/TALK  +  with/to THE LISTENER(optional) +  about TOPIC (optional)

Differences between speak and talk

Speak is more formal than talk. In a very formal context you may even hear someone use speaks to as an alternative to indicates, as in the example below. Talk to is not used this way.

• The fact that there are still millions of people out of work speaks to (=indicates) the seriousness of the recession.

In addition, talk is more likely to be used to refer to verbal interaction as an extended activity: communication that lasts a long time and covers a variety of topics:  

• They stayed up late and talked for hours.
• She never talks to me anymore.
• We need to talk.

Finally, only speak can be followed by a language:

• My grandparents spoke Yiddish. ("My grandparents talked Yiddish" sounds awkward and unnatural.)

I hope this helps.