He thought her to be a harlot: the word harlot is first used in chapter 34, when Dinah’s brothers had taken their revenge on Shechem. See 34.31 for some comments about translation of the term. There is the situation in some cultures, however, where prostitution as a custom is not recognized and where there are no regular terms for “prostitute.” This does not mean that women and men don’t engage in irregular sexual activity, or that money doesn’t change hands in the process; but it does mean that there are not the technical terms to render the Hebrew text at all literally. The following restructurings from one translation in which there is no term for “prostitute” may be helpful to other translators in similar situations:
In verse 14 Tamar takes off her widow’s garb and puts on other clothes “to attract men.”
Judah comes along in verse 15, sees the woman wearing those clothes, and is “interested.” He says to himself, “Hello! This woman looks as though she wants a man!”
In verse 20 Judah sends his friend to find “that woman.”
In verse 21 Hirah asks the men, “Where is that woman who always sits beside the road and waits for men?”
The men reply, “We don’t have that kind of woman here who is always waiting for men!”
In verse 22 Hirah reports back to Judah: “They told me that they don’t have that kind of woman there.”
In verse 24 the report about Tamar is that “your daughter-in-law has been looking for men and is in the family way.”

For she had covered her face: see discussion in verse 14. There are two possibilities for how this clause is related to thought her to be a harlot:
(1) Judah took her to be a prostitute because prostitutes commonly wore veils. This is the way ordinary readers of most English versions understand the text. However, commentators do not agree about whether it would have been true in this context that a veiled woman would most likely be a prostitute.
(2) Judah could not see who the woman was because of her veil, and so he assumed that she was a prostitute. In this case he would have to think she was a prostitute for a different reason: perhaps because she was sitting in that place, or perhaps because of something else about her clothing. One translation that follows this interpretation renders “wrapped herself up” in verse 14 as “she put on clothes to attract men,” and says here “Judah saw those clothes and thought….” Another possibility is “Judah saw her sitting there beside the road, and so he thought she was a prostitute. He didn’t recognize her because she was wearing a veil.”

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .