Rasmey Pen, a 15-year-old Cambodian native, struggled with the six jumbled words on the classroom blackboard, trying to figure out the one sequence that would turn the verb, nouns and pronoun into a coherent sentence.

He failed twice, blushing as his Montgomery Blair High School classmates snickered at him. He was correct on the third try: "I don't eat hamburgers with onions." Beaming, he turned to a Haitian student named Fan-Fan Prinston and shared a "high five" handshake.

Pen is only one example of a phenomenon that is slowly changing the face of Montgomery County's public schools. While overall enrollment in the system has declined from 126,000 in 1972 to 92,000 this year, the number of foreign students with little or no English skills has steadily increased, from about 5,000 in 1975-76 to 8,000 during the 1981-82 year.

The figures mirror national trends in public schools, according to statistics from the Education Department's Office for Civil Rights. The number of Hispanic and Asian students increased at the same time enrollment in U.S. public schools dropped from 43.7 million in 1976 to 39.8 million in 1980.

In 1976, there were 2.8 million Hispanic and 535,000 Asian-Pacific students classified as nonresident aliens. In 1980, there were 3.2 million Hispanics and 750,000 Asian-Pacific students.

The number of students receiving English for Speakers of Other Languages (ESOL) instruction in Fairfax County's public schools has grown from 1,900 in September 1980 to 3,500 this year, said ESOL coordinator Esther Eisenhower. And English is the second language to almost 25 percent of the 14,500 students in the Arlington County public schools.

In Montgomery County, there are children from 115 countries who grew up speaking everything from Farsi to Urdu. Some are the children of well-heeled foreign diplomats or representatives of international organizations while others are illegal aliens. Some are functionally illiterate and received no formal education in their own countries. Still others were brought here to escape the violence in nations like Afghanistan, El Salvador and Cambodia.

"I'm sure that few people in the county realize the problem this school system faces in trying to get children who come from 115 language backgrounds equipped to learn in English," says Kenneth K. Muir, director of information for the Montgomery County schools. "Sometimes you even have to teach them how to hold a pencil," Eisenhower adds.

Once there were so few foreign students in Montgomery that a small number of ESOL instructors drove to and taught at different schools around the county each day. But the increased need for intensive English instruction has caused the school system to raise the number of its ESOL teachers from only 29 in 1977 to 85 this year.

Now, the school system's budget for ESOL instruction has increased from $1.1 million in fiscal 1980 to $2.3 million this year. There are bilingual courses, language labs, an international students office that processes and rates each student's English level, and four high school intensive language centers for children who know too little English to grasp regular classroom instruction.

"When there is a problem somewhere in the world, we get their children in Montgomery County," says Betty Knight, coordinator for the school system's ESOL programs. "We got Afghans during the war there. We got Iranians in droves during the revolution there."

Statistics from the county's International Students Office show that Hispanics make up 32 percent of the county's foreign-born students. Last year, for example, there were 356 children from El Salvador, 113 from Nicaragua and 102 for Guatemala.

In their pre-high school years, foreign born students take most of their classes with American students and one or two ESOL classes where they receive intensive English instruction. The problems these students face in adjusting to American classrooms are numerous.

"Maybe a child was very smart in his own country. Here, he is the instant dummy. It's very stressful for them," says Knight. Frank Fernandez, supervisor of the International Students Office, adds: "The first three months are horrible for them. They sit in class and understand basically nothing."

One Korean girl became so obsessed that she frequently stayed up all night with a dictionary and eventually suffered a nervous breakdown. Another had the opposite problem, adjusting so well to American life that it angered her parents and she wound up running away from home.

"Sometimes the parents find it harder to adjust, as in a Vietnamese family where the father really runs the household," says Robert Talbot, coordinator for Northwood High School's intensive language center. "It's the children who are picking up the language and the parents lose face because they become dependent on their kids."

Shakera Nazari, a shy 18-year-old who just graduated from Northwood High, was one student who faced several problems when she arrived here from Afghanistan to join her father, a Washington restaurateur, four years ago.

"I cried everytime I went to school," Nazari says. "The American students never tried to help me. I went to ask directions once and they said 'That's your problem. If you can't speak English then go home.' "

But most of the foreign students, even those from war-ravaged countries, seem to adjust well to their new surroundings, say Knight and Fernandez. "You would never know what these kids have been through by looking at them," says Talbot. "They all look so innocent . . . I am very surprised at how well many of these kids have adapted."

Leslin Miranda, an 18-year-old fron San Miguel, El Salvador, talked one day through an interpreter about coming home from school in El Salvador and seeing a dead body in the street near her home and a neighbor moaning about her murdered son. The most difficult adjustment for her, she says, has been going to a coed school where boys are interested in talking to her.

Miranda sits in the most basic ESOL class, for students who know only a few nouns and cannot form English sentences.

In such classrooms, instruction often takes the form of a rudimentary "charades" game with the instructor mimicking the definition of a word. Every part of the classroom becomes a tool of instruction. Every object, from the clocks to the bookcases, have a sign on them with the English word written on it.

When one looks at how foreign born students perform on standardized tests and after graduation, the results are mixed at best.

While whites, blacks and Asians showed improvements in the Montgomery system's most recent standardized tests, comparisons between the 1980 and 1981 test scores for Hispanics showed an even mix of advances and declines.

Eleventh-grade Hispanic students tested in 1981 showed higher scores than their 1980 counterparts in eight of 11 test categories involving skills like spelling and language mechanics. But fifth-grade Hispanic students tested in 1981 showed lower scores in every category when compared to the 1980 scores, say school system statistics.

The school system is currently conducting a study to find out how well its foreign students do after they leave the school system. Some seem to do well, while others find themselves in menial jobs.

Nazari, the Afghan student, wants to go to nursing school, but will have to pay her own way and has been unable to find a job.

Eighteen-year-old Luis Alvarez has a different story. He attended a private school in Managua, Nicaragua, and came to this country last September. He entered as an 11th-grade student, knew only a smattering of English and "was so afraid," he said. "They hate Americans in my country and I thought the students wouldn't like me."

By the summer Alvarez was taking an English course on Shakespeare. He graduated a year early and plans to enter Montgomery College in January to learn computer programming.