Learn Hardcore Japanese: watasi ha tokidoki odoroita kao wo suru.

In Japanese, verbs can directly modify nouns. 驚いた顔 literally means “a face that got surprised,” which in English is “a surprised face.” The past form here describes the resulting state of the face, not past time. It’s a very common pattern for expressions: 困った顔 (a troubled face), 怖い顔 (a scary face), etc. You can also say 驚いている顔, but 驚いた顔 is the default and sounds more idiomatic.

Why can’t I say 驚く顔?

驚く顔 would read as “a face that surprises (someone),” i.e., the face is doing the surprising. You want the face to be the one that has become surprised, so Japanese uses 驚いた顔 or sometimes 驚いている顔.

What does 顔をする literally mean? Why the particle を with 顔?

顔をする literally means “to do/make a face,” i.e., to put on a certain facial expression. The を marks 顔 as the direct object of する. This is part of a broader pattern where certain nouns + をする mean “to do/have/put on [noun]”:

Facial expressions: 変な顔をする, 真剣な顔をする, 怖い顔をする, いやな顔をする.
Accessories/appearance: 指輪をする (wear a ring), ネクタイをする (wear a tie), 香水をする (wear perfume).
Activities/occurrences: 話をする (have a talk), けがをする (get injured).

Could I just say 驚く or びっくりする instead of 顔をする?

驚く/びっくりする = “to (actually) be surprised.” It describes your internal reaction.
驚いた顔をする = “to make/put on a surprised face.” It describes your outward expression and can be intentional or even pretend. Examples:
ニュースを聞いて驚いた。 I was surprised when I heard the news. (real reaction)
冗談で驚いた顔をした。 I made a surprised face as a joke. (expression)

Why is it する and not している? What’s the difference between 驚いた顔をする and 驚いた顔をしている?

驚いた顔をする: habitual or repeated behavior, which matches ときどき (“sometimes”).
驚いた顔をしている: describes a current ongoing state: “is wearing a surprised expression (right now/in that scene).” If you want a habitual sense with the progressive, you can say ときどき驚いた顔をしていることがある (“There are times when I have a surprised look”).

What’s the difference between 顔をする and 顔になる (e.g., 驚いた顔になる)?

〜顔をする: you put on an expression; it’s something you (can) do or adopt.
〜顔になる: your face becomes that way (a change of state). Examples:
彼は冗談で怖い顔をした。 He made a scary face (on purpose).
その音を聞いて、彼は驚いた顔になった。 On hearing that sound, his face turned surprised.

Why is は used after 私? Could I use が?

私は… sets “me” as the topic/background. It’s the neutral way to start a general statement about yourself.
私が… marks “I” as the focused subject, often used to identify or contrast: e.g., answering “Who is it that sometimes makes a surprised face?” → 私がときどき驚いた顔をする。 In most contexts here, は is the natural choice.

Can I drop 私 in this sentence?

Yes. Japanese often omits the subject when it’s obvious from context. ときどき驚いた顔をする。 is perfectly natural if it’s clear you’re talking about yourself.

Where can ときどき go? Does it need a particle?

ときどき is an adverb, so no particle is needed. Common placements:

私はときどき驚いた顔をする。 (most typical)
ときどき私は驚いた顔をする。 (fronted for emphasis on “sometimes”)
私は驚いた顔をときどきする。 (adverb between object and verb; also natural) All are acceptable.

Is びっくりした顔 different from 驚いた顔?

They mean the same thing. 驚いた is a bit more neutral/formal; びっくりした feels more casual/spoken. Both are natural: びっくりした顔をする / 驚いた顔をする.

How can I say “a face that looks/seems surprised” instead of stating it directly?

Use hedging expressions:

驚いたような顔をする = make a face that seems as if it’s surprised.
驚いたみたいな顔をする = same meaning, more casual. These add “it looks like” rather than asserting the state.

Can I use 見える to say “I sometimes look surprised (to others)”?

Yes. ときどき驚いて見える。 means “I sometimes look/seem surprised (to people).” This describes others’ perception, not your intentional action. You could also say 人からは驚いて見えることがある。

Could I use 表情 instead of 顔?

Yes, but collocations differ. You can say:

驚いた表情をする (used, somewhat formal), or more idiomatic:
驚いた表情を浮かべる, 驚いた表情になる, 驚いた表情を見せる. Using 表情 shifts focus to “expression” in a slightly more descriptive or formal tone.

How do I make this sentence polite, negative, or past?

Polite: 私はときどき驚いた顔をします。
Negative: 私はめったに驚いた顔をしない。 (hardly ever) / あまり驚いた顔をしない。 (not much)
Simple past (one instance): 私はときどき驚いた顔をした。 (context needed)
Past habitual: 昔はときどき驚いた顔をしていた。 (“I used to sometimes…”)

Why are there spaces between the words here? And can ときどき be written in kanji?

Japanese normally doesn’t use spaces; textbooks often insert them for learners. The standard writing would be 私はときどき驚いた顔をする。 Also, ときどき can be written 時々; both are common with no real nuance difference.